Mailcap files, defined in RFC 1524, allow you to tell you mail clients, web browsers, and other programs how you want to view and edit various MIME types. Since interaction with your mailcap files often occurs deep within the bowels of your program, I've written up a very simple Python script to test your mailcap entries: mailcap-test.py. Enjoy!
Related utilities include xdg-open, mimeopen, and sensible-utils.
Quoting
Quoting in mailcap files is a tricky issue. From the Mutt man page:
Secure use of mailcap
The interpretion of shell meta-characters embedded in MIME parameters can lead to security problems in general. Mutt tries to quote parameters in expansion of
%ssyntaxes properly, and avoids risky characters by substituting them, see themailcap_sanitizevariable.Although mutt's procedures to invoke programs with mailcap seem to be safe, there are other applications parsing mailcap, maybe taking less care of it. Therefore you should pay attention to the following rules:
Keep the %-expandos away from shell quoting. Don't quote them with single or double quotes. Mutt does this for you, the right way, as should any other program which interprets mailcap. Don't put them into backtick expansions. Be highly careful with eval statements, and avoid them if possible at all. Trying to fix broken behaviour with quotes introduces new leaks - there is no alternative to correct quoting in the first place.
If you have to use the %-expandos' values in context where you need quoting or backtick expansions, put that value into a shell variable and reference the shell variable where necessary, as in the following example (using $charset inside the backtick expansion is safe, since it is not itself subject to any further expansion):
text/test-mailcap-bug; cat %s; copiousoutput; test=charset=%{charset} \ && test "`echo $charset | tr '[A-Z]' '[a-z]'`" != iso-8859-1