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)