We've been round this loop a few times over the last year or so!
So here the usual questions;
What format and size is the file?
Hi-Res video can quickly reach several 10s of MBytes or more.
The MB site will only accept very short video clips, of less than 5MByte I think.
The only way to post large files, of several minutes length, here is to first upload it to You Tube, and then put the URL link to it in your post here.
See the box on the right of the post editing window.
That works for me on a PC.
If you are using the Mobile App you'll have to ask Stephen (Fireboat).
How precisely are you trying to do it?
Cheers, Doug 😎