ShareFile Upload Failures

I use ShareFile for file sharing with clients.  Some files I upload subsequently fail with the same distinct pattern of getting to about 10% stopping for a period and then restarting for 4-5 times before failing with non-descript "network error".

My network is fine.  It's always certain files that fail (PowerPoint with embedded video file and zip files of various data file types).  File sizes vary from less than 100Mb to 1Gb so it's not any sort of network limit.

ShareFile representatives tell me it's the firewall causing the issue after reviewing error messages found in Developer Tools while attempting upload.

Please advice to what settings in the firewall may be blocking these files and/or suggest troubleshooting.