I had the same issues too years in a row and also found help line to be very unhelpful and uninformed. I just had this problem and after reading some comments, realized that my file sizes were too big. When I converted my jpeg file of Form 8453, it was a 19 mg PDF file. I then printed my jpeg as a 'print as image', which still converts the jpeg to a PDF, the PDF file was under 1 mg. I then re-attached the new and smaller 8453 PDF file and it went through immediately.
