Default file extension.

[Phylis Sophical]

using version 2.6.5

I am editing home movies, cutting them into smaller video's, removing black frames, etc. I thought when I started this project that it automatically saved as .avi. But now, there is no default file extension and I have to put that in when saving the section.
Is there a way to have it just automatically save as .avi ?
My original home movies are in .mp4 format. So would it be better to save these sections as .mp4's?
If so, would I have to make any other changes?
I've saved some as .mp4 and there doesn't seem to be much difference.

Thanks for any suggestions.