Please add the option to Automatically add File Extension According to the Container used.
Normally avidemux does this. What is your used OS en version of avidemux? Here avidemux adds mp4, mkv, avi and .ts extensions to newly created videos. This is already some time present
I am using Avidemux 2.5.6 on xp sp3 and it has never added extension automatically. I mostly Use Avidemux 2.5.6 to cut avi file because Avidemux 2.5.6 has the smart copy feture which Avidemux 2.6 is missing so i don't use Avidemux 2.6.
Current avidemux 2.6.3. And I have 2.5.6 alongside 2.5.6: for the few cases 2.6.# can't handle. By now you should be aware of the big differences between 2.5 branch and 2.6 branch: 1st is frame based editor, 2nd is time based, these don't mix very well. Don't know if in future these 2 different ways of operating ever can/will merge. Due to the lack of time I don't see much happening in the 2.5 branch. My opinion as user and may not reflect the developer(s) view on these.