File::Basename and a hash like

%extensions = ( 'c' => 'C Source',
                'cpp' => 'C++ Source',
                'pl' => 'Perl Script',
                'dat' => 'Data (Text?) File',
                'csv' => 'Comma Separated Values',
                'txt' => 'Text'
               );
[download]

are what I would use. If you can't trust people to use the extensions appropriately, there is no other Unix way to differentiate (that I know of). Files are either binary or ascii (Update: AgentM has a good point below, and I don't want to belabor the issue). End of story. If you are intrepid, I suppose you could check the C source for things like #include <stdio.h> just for kicks, but I wouldn't rely on that any more than I would rely on #!/usr/bin/perl to indicate a Perl script.

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