Uploads of special JPGs as attachments trigger a debug-message:
While creating a totally black preview-picture, the JPG-file itself is uploaded without any problems.
I assume that this error only occurs in 1 from 1000 JPG-files, I can send the original to a team-member privately for inspection.
A total description of this matter is posted in support-forum:
BTW: This same debug-message also occurs on phpbb.com by trying to upload this file.