Hi Marissa Dimino,
For information on supported file types, please review the following: https://community.jivesoftware.com/docs/DOC-37679#jive_content_id_What_formats_are_supported_for_uploads .
Since you are seeing an error message, you'll want to open a formal Support case within you secret JiveWorks Group. After that, a member of Jive Support will work with you in confidentially to troubleshoot the issue.
We're experiencing this problem too. It seems to be happening only with mp4 files at this point. We've tested a few other file formats. It happens both when creating a video and when uploading into a blog post. I'm going to open a case, but wanted to share here as well.