My customer was attempting to edit a teammate's Comment on their space, as his Attachment (an .xlsx file) did not show up as clickable. The customer added the attachment again just below (and within) his teammate's original comment, saved it and received an error of sorts (see attachment). He tried adding a brand new comment, this time with a .png file from their Attachments list for the Space, and the same thing happened. They ended up with a line that noted the file name, a replacement name before it, and the line was highlighted red.

The customer tried this same method on a Test Space we have on Confluence, and received no error. I also tried this on their Space, as well as the Test Space and encountered the same results.