Comment 12 for bug 1280794

With .mp4 can add another that will cause this - video/x-m4v
So for example if a user sets Videos as default thru "Default Applications" then these 3 are entered in mimeapps.list
video/mp4=
video/mp4v-es=
video/x-m4v=

Nautilus only sets the first one so if the other 2 exist then the default for .mp4 cannot be altered thru nautilus until they are removed or edited to intended new default.

Also if another player is set thru Default Applications & the above 3 are in mimeapps.list then the default for .mp4 will only be changed if that player has all 3 in it's .desktop's MimeType= line