Comment 0 for bug 1041144

If you want to associate files with applications not listed in the 'Properties'->'Open with' list, you can click a button below to display more of them. However, when you click it, Avidemux-GTK is not among the listed applications.

The following fix made it appear listed and selectable:
http://askubuntu.com/questions/82652/file-association-to-unlisted-application