I’ve created six files with bogus extensions. The filename prefix denotes the actual content (text/plain, image/png and application/octet-stream), and I simply tacked on an extension for decoration. What happens when I send off a message with all six files attached? Let me look at three e-mail clients I use regularly. Lotus Notes 7.0.2 on Mac OS X displays the problem right after attaching the files. Note how the icons match the file extensions and not their content (remember: the file’s content are the first three letters of the name, the rest is decorum): Not a single one is right. In Thunderbird version 2.0.0.6 we have a very similar situation after attaching the files: All wrong again. Mutt interprets the files with these content-types and gets none right. The reason is explained in the manual: Mutt consults the mime.types file to determine the type from the extension.

A  2 exe.jpg                 [image/jpeg, base64, 56K]
A  3 exe.txt   [text/plain, quoted, unknown-8bit, 56K]
A  4 png.jpg                [image/jpeg, base64, 133K]
A  5 png.txt  [text/plain, quoted, unknown-8bit, 133K]
A  6 txt.exe        [applica/x-msdos-pr, quoted, 0.1K]
A  7 txt.jpg                [image/jpeg, quoted, 0.1K]

Is it trivial to determine a file’s content? Let me use the Unix file command and see.

$ file ???.???
exe.jpg: MS-DOS executable (EXE), OS/2 or MS Windows
exe.txt: MS-DOS executable (EXE), OS/2 or MS Windows
png.jpg: PNG image data, 500 x 778, 8-bit/color RGB, non-interlaced
png.txt: PNG image data, 500 x 778, 8-bit/color RGB, non-interlaced
txt.exe: ASCII text
txt.jpg: ASCII text

Yes, it is trivial.