I belong to a forum [Removed as per Community Guidelines] and recently encountered a problem uploading a .mov file to a thread. The file uploaded but there was no video, only audio. I converted the file to .mp4, uploaded and all was well. I checked with someone else on the forum and he was able to upload .mov files and they played; video and audio.
My son sent me the files last year (from his iPhone 12), uploading from his USB-C stick to DropBox. On the phone and stick they are the standard file names but after the upload, DropBox changed the file name to a date/time stamp. I told my son and he thought it was unusual but nothing more. The files played fine on my laptop (Lenovo, 2021). After several tries at uploading and getting the same results I downloaded someone else's .mov file that had been posted. I renamed the file and uploaded it. No problems, the video played fine.
So if the file names had been modified, perhaps the files themselves were modified too??? I have no other explanation and I'm not sure where in the process the names were changed, but my son says they are still the original names on the phone and stick.