This is similar to bug 8222, but greater in scope. The supported mimetypes list in gst-thumbnailer is outdated, and has not been synced with Totem in a long time. Due to this, a number of media files are not thumbnailed.

The current supported list is:

    "application/asx",
    "application/ogg",
    "application/x-flash-video",
    "application/x-ms-wmp",
    "application/x-ms-wms",
    "application/x-ogg",
    "video/3gpp",
    "video/divx",
    "video/flv",
    "video/jpeg",
    "video/mp4",
    "video/mpeg",
    "video/ogg",
    "video/quicktime",
    "video/webm",
    "video/x-flv",
    "video/x-m4v",
    "video/x-matroska",
    "video/x-ms-asf",
    "video/x-ms-wm",
    "video/x-ms-wmp",
    "video/x-ms-wmv",
    "video/x-ms-wvx",
    "video/x-msvideo",
    "video/x-ogg",
    "video/x-wmv",

The supported list for totem is at: 

http://git.gnome.org/browse/totem/tree/data/mime-type-list.txt

The actual mime type list is generated by running "./mime-type-include.sh mime-type-list.txt > totem-mime-types.h". Perhaps Tumbler should do something similar? If not, the current list should at least be synced with the totem list. :)