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.