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Thomas Seifert
if it would be only the image part I'd have an idea but as you say this is for all files are hit its getting tricky. You might try the change below anyway.
I just did 2 test attachment uploads.
I uploaded a jpg photo and the image is broken. (embed attachments, embed images).
I uploaded a pdf and the link is displayed properly in the message text (embed attachments) and can be clicked and downloaded.
It's definitely an image rendering issue.