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From experience I know that if I save .jpg file with an .png extension (or vice versa) the most programs will open it as normally. I am wondering why is that the case and ask people with experience in the video codecs, what will happen if I try to save mov or avi files as mp4? (To be totally clear: By saving I mean, renaming their filename with the non-corresponding extension)

Will the video players supporting AVI and MOV be still able to play the file if its file extension is MP4.

  • If so what are all the possible issues you could describe that could pop while trying such a playback?
  • If not, why? And also why this is possible with images? Describe all the technical details you feel related to this topic!

Thanks :)

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    Note that in the case of an image viewer, there are parsers and renders written into the program to handle all the appropriate types and their code does not determine which type it is just based on the ext, as Tonny explained. you could not for instance display a word document in a image viewer though, since that programmer has no logic for dealing with the document format. – Frank Thomas Aug 7 '20 at 20:46
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    You may be interested in the file program. – chrylis -cautiouslyoptimistic- Aug 8 '20 at 2:32
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    A side note: Some file types, like AVI or MKV, are container formats that have to be parsed anyway and can be encoded in a variety of ways. Even a program that can handle AVI may not be able to open all AVI files, correct extensions or not. Results can be frustratingly inconsistent when testing any of these mix/match scenarios. – Booga Roo Aug 8 '20 at 2:50
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    These days mov and mp4 are different names for exactly the same format. The others mentioned are actually all different formats. – OrangeDog Aug 9 '20 at 10:50
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Most programs don't look at the extension AT ALL. They look at the file-header content to determine what it really is and act accordingly.
Almost every well-known standard file-format has recognizable identification in the first bytes of the file. (E.g Every GIF image has the characters "GIF87a" as the first 6 bytes.)
If the software knows how to handle it, it just does (some do give a warning that the extension is wrong), if it doesn't it gives you an error message (or just crashes if it is badly programmed).

The extension mainly serves as a visual indicator for you indicating what the file most likely is.
And it allows your OS to quickly determine what application is best suited to handle it, without having to actually read the content of the file.

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    In my experience, this usually only applies to binary format files. Would you say this is your experience as well? I mean the only real indication that a text file is a JS file or just file full of cat names is the extension. Sometimes you have to try parsing and failing before knowing. – zero298 Aug 8 '20 at 15:49
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    some gifs start GIF89a – Jasen Aug 9 '20 at 8:00
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    @yyny Technically that is correct, but for most people there really isn't a difference between "the OS" and 'the File/Desktop Manager". In fact: Most people don't even know they have a desktop manager. I didn't want to complicate the answer too much :-) – Tonny Aug 9 '20 at 10:19
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    Demarcating file managers and OS for Windows is pedanticism - for probably 99% of people using Windows, their file manager is Windows Explorer, and for all those people the extension definitely does affect how a file is treated. – Hashim Aziz Aug 9 '20 at 18:04
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Changing the name of the file does exactly that: change the name of the file. Nothing more. In particular, changing the name of the file does not change the content of the file, only the name and nothing but the name.

(In fact, changing the name of the file will actually not touch the file at all, since the "name" is actually just an entry in the directory. It isn't associated with the file.)

Since nothing about the content of the file itself changed, it should not be surprising that a program that was able to correctly decode the content of the file when it was named Fred will also be able to correctly decode the content of the file when it is named Wilma, for the simple reason that the content is exactly the same.

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    It definitely is associated with the file, it's just the file contents are stored separately to the file metadata. – OrangeDog Aug 9 '20 at 10:46
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    @OrangeDog I suppose what OP is saying is that the filename is associated with a directory entry, which in turn is associated with the file. The filename itself isn't directly associated with the file, compared to, for example, permissions, timestamps, ownership, etc. in ext4 on Linux, where two hardlinks might have different names, but still share those other attributes. – muru Aug 9 '20 at 17:13
  • Obviousky it can decode it but many systems still use extensions to determine app association. – RichieHH Aug 18 '20 at 12:38
  • @muru (1) You might be right in that supposition in that it might indeed be what the OP had intended to express, but it factually isn't. if the file meta data is associated with anything but itself, then it is with what it exists to describe, then it is that very region of persistent storage. (2) Whether physically a (slightly) different location or not, not only users are presented both the file and it's meta data in conjunction usually, so are OS APIs and by proxy userland applications. – Johannes Pille Aug 30 '20 at 9:02
  • (3) What are you even saying? "The filename itself isn't directly associated with the file, compared to, for example, permissions, timestamps, ownership" Come again? "The file" in this context surely is it's content itself, everything else is just meta, no? All the data you mention, beginning with "For example permissions" are stored on an inode, which is distinctly and deliberately not the file itself. Bunch of half-truths and misinformation you're spreading here. – Johannes Pille Aug 30 '20 at 9:42
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Almost all file formats embed information about what the file type is right near the beginning of the file. For example, a real PNG file always starts with the eight bytes 0x89 0x50 0x4E 0x47 0x0D 0x0A 0x1A 0x0A (note that bytes 2 through 4 are the ASCII characters 'PNG', the rest of the header is binary data designed to detect the file being handled in ways that would result in data corruption), or an ELF object file (used for executables on most systems other than Windows and macOS) starts with 0x7F 0x45 0x4C 0x46 (with bytes 2-4 being 'ELF' in ASCII). These are known as file signatures, and while they are not the only way to determine a file type based on contents, they're generally the first step. Wikipedia has a list of them for many common file types that may be of interest.

The near ubiquitous use of file signatures means that you can look at the contents of the file itself to figure out what type of file it is, and almost all software does exactly that for two reasons:

  • It's significantly more reliable than matching on file extensions (or on the MIME type reported by the server you're downloading the file from) because you can't modify this data and still have it be a valid file of that type but you can change the extension (or MIME type) to whatever you want and the file will not change what type it is.
  • Validating the file type is an important layer of protection against crashing the application or exploiting bugs in it. If you blindly trust other sources of information about the file type, you run the risk of trying to parse something as one type of data when it is in fact a different type, which can cause all kinds of problems. Insufficient validation of the file structure given the expected file type has historically been a very common attack vector for malware.

Windows is largely the anomalous case here in that it predominantly favors file extensions over actual file contents for deciding what to tell users the file type is, while most other systems and most applications only fall back to the file extension if they can't figure out the type by looking at the file contents. The sole practical purpose of a file extension these days is to act as a generic indicator of what the file type might be, making it easier to determine what type of file you're dealing with or find files of a specific type without having to inspect the file contents, though in some cases people just choose to inspect the contents anyway (see for example the file command from UNIX systems).

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  • It's also quicker to check the file extension, than to open and read the file looking for a signature that may not even be there. – OrangeDog Aug 9 '20 at 10:48
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    Upvoted, but beware of repeating the common mantra that "Windows uses file extensions, other OSes don't". As I tried to explain here, both file extension and file contents are generally being looked at by particular programs, and a lot of programs (on any OS) will use the file extension because it's much more efficient, and in some cases actually more informative, than the available alternatives. – IMSoP Aug 9 '20 at 11:48
  • @IMSoP Windows itself (not any applications, but the actual OS and user shell) does indeed look at file extensions to the exclusion of contents in most contexts where it's presenting file type information to the user, while macOS and all major UNIX environments preferentially look at contents (or the resource fork in some cases on macOS and similar systems like BeOS). I'm not referring to cases where you're actually opening the file here, just presentational behavior. – Austin Hemmelgarn Aug 9 '20 at 12:08
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    @AustinHemmelgarn There is no agreed definition of "actual OS", and plenty of examples on all OSes where both names and contents are used to make decisions about the file. The statement "Windows Explorer (the default file manager in Windows) uses the file extension to choose icons and default actions" is true; the statement "Windows programs have a stronger tradition of relying on the file extension" may also be true; but the common characterisation of "Windows does one thing, other OSes do the other" is misleading. – IMSoP Aug 9 '20 at 12:17
  • @IMSoP My point that the presentational aspect on Windows prior to the user actually opening the file almost exclusively cares about the file extension is still the case though. I will agree that I could have worded my statement more clearly to express this (and have now done so). – Austin Hemmelgarn Aug 9 '20 at 12:24
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In general, a file extension is a way of providing a clue to some piece of software about what format the file's contents are in. The other clue that's usually available is the contents of the file itself, which often include an explicit header at the beginning of the file for this purpose.

Every piece of software is free to use either or both of these pieces of information. Some common approaches are:

  • Ignore the file extension completely, and just examine the file contents.
  • Ignore the the file contents completely, and just look at the file extension. Error if the file can't be processed as that type.
  • Look at both, and warn the user if they don't match, then attempt to process according to the file contents.
  • Look at the file extension initially, and attempt to process as that type. If processing fails, warn the user and try to guess the type from the contents, or even ask the user.

Changing the file extension will therefore affect different programs differently:

  • It will change the icon and double-click action in Windows Explorer, and change the download behaviour on an Apache web server, because those programs look at the file extension only.
  • If the file is not valid for the file extension you select, it may cause some programs to refuse to open the file.
  • If the file can be validly interpreted as more than one format, changing the file extension may cause some programs to change how they process it.

For videos in particular, most file formats are "containers" anyway, so have a lot of metadata at the start of the file to indicate exactly how they have been encoded and assembled. It's therefore likely that software for working with them will take a content-first approach, and changing the extension will either make no difference, or give a warning and then proceed as normal.

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im fairly sure i have tried the same video renamed with 2 or 3 different extensions on one of those media player boxes or dvd player, and some files played ok & not others - exactly the same file only differing in extension

and irfanview will tell me if a jpg is really a gif or vice versa & offer to rename it for me

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  • Media players, smart TVs etc. can be particularly finicky about the media they will play... requiring it to be in a specific format and named just so. – DocRoot Aug 10 '20 at 16:27
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I'd just like to add that web server software in particular associates file extensions with specific MIME types, which are then used to send the file to the user's web browser software. Depending on how the user's browser is configured (display the file in the browser; use a plug-in or external app; save the file) you could get some "improper" results by using the wrong file extensions.

For example, it's possible to have the the binary data making up a GIF or JPG file displayed as text, or downloaded as a file, if sent with a text/plain MIME header, as a result of a .txt file extension on the image file. File extensions are mapped to specific MIME types in a config file, typically apache_home/conf/mime.types although some newer versions use a "magic" file instead that are able to detect file types by describing file header metadata, in the same way that the UNIX/Linux "file" command works.

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File extensions are generally used (in Windows) to work out which program to use to play (or otherwise process) the file, by means of "associations" which you can see and change in Settings > Apps > Default apps. If you have more than one video player installed on your machine and you click a video file in File Manager to open it, this is how the operating system decides which player to use. If you don't like the automatic choice, you can right-click and use "Open with" to pick a different app.

A similar thing happens with MIME extensions in web browsers. But Linux-like operating systems typically don't do this - they look at the file content rather than the file name to work out how to decode it.

If the file extension is wrong and the file is therefore opened by a program that can't handle it, you will get some sort of error message that is specific to that program, or the program might even crash, or infect your machine with a virus.

Your question may be assuming a different situation though, where the video player is started first and then the video is selected from within that player using a method such as File > Open. In that situation, the file extension is often (though not always) completely ignored and the program will work out how to play it by examining the file content. Again, if it doesn't recognise the content it will typically give an error or some other unpredictable behaviour.

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The extension does not determine the file type. It is a single observation of the file. You should use programs like "file" (in linux) to determine the file type.

If I can read the "A" or "B" format and there is a "A" format with "B" extension, I will be able to open it :)

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It will be a generally bad practice to use structural features - like file- or directory- or volume- or hostnames - to store meta-data - like date/time, order or even the kind of content inside.

But: regardless the widespread usage of this really smelling practice ;)

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  • I sort of get what you're saying, in a very abstract way, but ultimately file names are always metadata - if I name a file "Sales Report for Mark", I'm adding metadata that will help me remember what it is, rather than having to identify it as "inode#35491057" or "{a8ce2701-44ad-4d14-befc-06239ef1b506}" or whatever. Some file systems (e.g. Apple HFS) have a separate field for file type; but then arguably so did FAT: the three-letter "extension" was intended for storing type information. There have been attempts to make more "database-like" file systems; none have become mainstream AFAIK. – IMSoP Aug 26 '20 at 10:46

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