The DEP-11 proposal to add AppStream information to the Debian archive, is a proposal to make it possible for a Desktop application to propose to the user some package to install to gain support for a given MIME type, font, library etc. that is currently missing. With such mechanism in place, it would be possible for the desktop to automatically propose and install leocad if some LDraw file is downloaded by the browser.
To get some idea about the current content of the archive, I decided to write a simple program to extract all .desktop files from the Debian archive and look up the claimed MIME support there. The result can be found on the Skolelinux FTP site. Using the collected information, it become possible to answer the question in the title. Here are the 20 most supported MIME types in Debian stable (Squeeze), testing (Wheezy) and unstable (Sid). The complete list is available from the link above.
Debian Stable:
count MIME type ----- ----------------------- 32 text/plain 30 audio/mpeg 29 image/png 28 image/jpeg 27 application/ogg 26 audio/x-mp3 25 image/tiff 25 image/gif 22 image/bmp 22 audio/x-wav 20 audio/x-flac 19 audio/x-mpegurl 18 video/x-ms-asf 18 audio/x-musepack 18 audio/x-mpeg 18 application/x-ogg 17 video/mpeg 17 audio/x-scpls 17 audio/ogg 16 video/x-ms-wmv
Debian Testing:
count MIME type ----- ----------------------- 33 text/plain 32 image/png 32 image/jpeg 29 audio/mpeg 27 image/gif 26 image/tiff 26 application/ogg 25 audio/x-mp3 22 image/bmp 21 audio/x-wav 19 audio/x-mpegurl 19 audio/x-mpeg 18 video/mpeg 18 audio/x-scpls 18 audio/x-flac 18 application/x-ogg 17 video/x-ms-asf 17 text/html 17 audio/x-musepack 16 image/x-xbitmap
Debian Unstable:
count MIME type ----- ----------------------- 31 text/plain 31 image/png 31 image/jpeg 29 audio/mpeg 28 application/ogg 27 image/gif 26 image/tiff 26 audio/x-mp3 23 audio/x-wav 22 image/bmp 21 audio/x-flac 20 audio/x-mpegurl 19 audio/x-mpeg 18 video/x-ms-asf 18 video/mpeg 18 audio/x-scpls 18 application/x-ogg 17 audio/x-musepack 16 video/x-ms-wmv 16 video/x-msvideo
I am told that PackageKit can provide an API to access the kind of information mentioned in DEP-11. I have not yet had time to look at it, but hope the PackageKit people in Debian are on top of these issues.
Update 2013-01-16 13:35: Updated numbers after discovering a typo in my script.