Some notes on MIME types in Gentoo linux (and presumably other distributions)...
MIME types
The app-misc/mime-types packages provides a list of mime types in /etc/mime.types (note that this package has a version number on the end - the exact version of mine is app-misc/mime-types-5. You can find it with equery list mime-types).
/etc/mime.types contains a mapping between mime types (e.g. application/msword) and document extensions (e.g. doc).
The mime types themselves are kept in /usr/share/mime/ and hold full details of the type (e.g. their natural language name).
Default applications
The list of applications used to open the mime types are kept in /usr/share/applications/ and are named [app_name].desktop. There is also a mimeinfo.cache file in this directory which contains a mapping between the mime types and the applications used to run them.
If you are having problems opening a file because the associated helper application doesn't exist, edit the mimeinfo.cache file so that it references the correct .desktop file.
