Post attachments: strange behaviour

Hi everyone,
when creating a video post and attaching an mp3 file to it (about 50MB), I don’t see the file being uploaded, instead I get a message from my browser (I am using Brave Browser, based on Chromium) asking me if I really want to close the page. If I leave the page, the attachment is not being uploaded, while if I remain on the page it is. But as I said there is no “uploading…” message, and there is no message when the uploading is finished, I have to make a guess and actually close the page to verify.
Is anyone experiencing this strange behavior too?

Cheers.