This should be fixed now. It was a timing issue. We upload the attachments to S3 and then query S3 for a list of attachments. But S3 hadn't finished whatever it needs to do. I've added a delay.I suspect you may also have to similarly-tweak another part of the code in regards to attachments and S3; I was actually in the middle of writing that bug report when I read this and it clicked.
I take it you are also using calls to S3 to query and retrieve (and embed/attach) the in-line image/attachments when the Follow-Topic catch-up emailer code is invoked.
All of a sudden*** the attached images in the FO catch-up messages, while correctly named as in the original mailing, are now nothing but a 212 to 214 byte (in my test case) gibberish-contents file. (I can attach/mail them offlist if you think it may help)
*** or at least I just noticed it while testing for another bug I also just found, in that same FO catch-up mailer area (to be submitted soon)