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I have a compressed file but I don't know what it's compressed with. The extension doesn't give any clue. How can I find out what it is?

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What extension do you have? – Gnoupi Dec 31 '09 at 10:30
I'm guessing he doesn't know the extension, cause it is renamed to something like 001, or whatever. – Rook Dec 31 '09 at 13:31
How do you know it's a compressed file? – RJFalconer Dec 31 '09 at 15:17
I know it's compressed because the person said so. The challenge is to figure out how to uncompress it. – Phenom Dec 31 '09 at 20:22
up vote 4 down vote accepted

As an alternative to inspecting the file header by hand, you could use some utility like TrID. The link points to the cross-platform command line version; for Windows there's a GUI, too.

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You don't say which operating system you're using.

If you were using a Unix-like OS, you could try

$ file filename*
filename.exe: MS-DOS executable PE  for MS Windows (console) Intel 80386 32-bit
filename.png: PNG image data, 2160 x 934, 8-bit/color RGB, non-interlaced
filename.rar: RAR archive data, v14, flags: Archive volume, os: Win32

where filename* is the name of the file(s) you want to check. Note that this program works by examining the file data, not the filename extension, so it would give the same output for filename.rar if you renamed it

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file is also available from many Unix-tools-on-Windows providers, including Cygwin and UnxUtils and possibly GnuWin32. added sample output to your example. – quack quixote Dec 31 '09 at 11:59

7-zip is known to open a very wide array of compressed files, you can try with it.

Also, you could check the file in some simple text editor, often the file type is written in the first letters. Maybe you could have more informations that way.

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Try the portable version of 7-zip if you don't want to install it. – Nifle Dec 31 '09 at 12:22
Already tried that. 7-zip doesn't recognize what it is. – Phenom Dec 31 '09 at 12:37

Use WinRAR - can you download a free trial version and it does not expire or stop you from using it after the trial is over.

To confirm this, I created a .zip file with .xls and .pdf files. After I removed the .zip extension from my file. I then opened my archive without an extension in WinRAR and it opened the archive and showed me the files in the archive.

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