Hi Nicholas,
Arbil assumes that you plan to archive with TLA and this means that all files you attach will be automatically checked for archivability. As you can see from the list of archivable formats (http://www.mpi.nl/corpus/html/lamus/apa.html), we do not accept m4v files.
Unfortunately there is currently no way to switch off this typechecking globally, you can only override it on a file by file basis.
But if they are normal mp4’s you can probably just rename them to .mp4 and this will make Arbil like them.