[@robin-w Moderator 5 years, 11 months ago]

bbpress itself does not allow uploads, so it will be an add-on plugin.

https://en-gb.wordpress.org/plugins/gd-bbpress-attachments/ is one of the favourites for that.

The actual error is almost impossible to work out without knowing what is on the site.

it could be file type, or file size, so give that some experience and other not, then I’d try uploading different types and different sizes to see if you can work out when this happens.