Well i struggled with this the other day so i thought i would post the solution i found.
when rendering the video I used QuickTime format and MP4 (MPEG-4 Video in adobe premiere) compression but the files still came out as .mov files.
I couldn't get them to be .mp4 files like istock wanted.
What I ended up finding out was that as long as the file is the correct format, you can just change the file ending from .mov to .mp4 and submit your test images that way. It is perhaps a work around but the test images went through today, so it seemed to work.
And for the search engine for those poor souls who are having the same problem as me
How to create an mp4 from mov for istockI can't create an mp4 filemp4 problem with upload - istock