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When I have a random file of unknown format with a file extension I've never heard of, where can I turn to find out about the extension. The obvious choice is to google the extension along with some combination of the words "file", "extension", and so on, but that just yields pages of spam websites peddling anti-virus ("Fix virus problems related to XYZ files!"). Is there any better way?

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You did not mention what OS, see this page for a software solution...mark0.net/soft-trid-e.html –  Moab Apr 22 '11 at 4:47
    
I was assuming that most methods would be OS-agnostic, but that does look useful. Why don't you post it as an answer? –  Ryan Thompson Apr 22 '11 at 7:45
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6 Answers

I suggest you give a look at Whotsit.org. There's no file extension/format I was unable to find on that site over the last 10 years or so.

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You could also look at the contents of the file, using a tool like the Unix file command.

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Could you edit your answer to provide a bit more information about how this might be helpful? –  nhinkle Apr 22 '11 at 3:49
    
The file command attempts to identify a file by testing the beginning of the file against a set of patterns for known file types. If it matches one, it will report the file type. Useful when it works, but there are lots of esoteric formats that it doesn't know about yet. –  Ryan Thompson Apr 22 '11 at 7:42
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Windows or Linux software solution

http://mark0.net/soft-trid-e.html

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The File Extension Seeker by file-extension.net is pretty comprehensive.

http://file-extension.net/seeker/

The Wosit.org site is good as indicated above but its scope is a bit limited.

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Wikipedia also tends to have a good collection of file extensions. Get used to using one site you trust and then google the unknown extension like this:

.ext site:trustedsite.com

Google also allows you to remove certain sites from it's search results for you only, though the sheer number of possible annoying sites could make that unreasonable for this particular issue.

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