Looking at the code, the only reason that happens is if the attachment file doesn't exist or if it's not readable by the web server. Presumably the user saw the image correctly after it was uploaded, then the attachment did exist at one point, so you'll need to look in your internal_data/attachments/1/ directory to see if you can find a file for the attachment ID 6 (it will have a hash attached to it).
If you have add-ons installed that affect attachments, that could cause issues like this. I would recommend disabling all add-ons and then uploading an attachment yourself and confirming that is exists and is viewable.