Is it possible to download youtube videos in mp4 format by default with importfeed. Without any options git annex often downloads the webm format.
git-annex uses quvi for these downloads. If quvi has a command-line
option to make it prefer mp4, you can pass it using the annex.quvi-options
git configuration setting. Or maybe it has a config file. I don't
know how to configure quvi to do that, but if it's possible to, you should
be able to get git-annex to run it with that configuration.