I think that message is another issue as to the previous one, that previous issue should as mentioned be rare.
Find, in Administration > Forum Settings > Allowed file types:
Default is : .gif,.jpg,.png,.zip,.rar,.tar,.7z
Separate each type with a comma
Some files might have a large ending, add for example ,.JPG and not .jpg and it can fail. Try to add some combos and see how it works.