What video formats are supported for uploading to the forum?

mp4 has worked for me in the past.

Here's an mp4 from [https://file-examples.com/index.php/sample-video-files/sample-mp4-files/]:

(I downloaded it locally first, in case there's any quirks with directly linking a video.)

What device, OS and browser are you using?

The video from the local file plays fine in my browser. It's after I upload it to the forum it doesn't work. A link isn't being processed by the forum backend.

Sorry I wasn't precise in my previous reply, but I meant that I uploaded a local file as opposed to a specifying a link to be uploaded, as the forum allows you to do attach images and videos either way (although sometimes uploading doesn't seem to work). I didn't mean literally pasting a link into a comment.

As a matter of fact, I actually uploaded the mp4 in the comment above using drag and drop (not Insert > Image/Video/File).

Do you have a copy of the video hosted somewhere? Obviously mp4s in general aren't broken. The mp4 above plays for me (does it play for you?) and I've posted mp4s in the past without any problems.

I tried a different video editor on my phone and it made a working copy that I posted on another thread. So, some editors are better than others.