Did I understand you correct:
An inbound e-mail that is HTML formated with an attachment will create a new ticket, but dosn't attache the attachment, but a rich-text or plain text mail with an attachment creates the ticket and the attachment is attached to the new incident?
I heard about something like this some time ago.
Has your HTML e-mail somewhere an URL included, for example in the signature?
I can remeber something, that there were or are problems with HTML formated e-mails containing an attachment and somewhere an URL link, so that the inbound mail will fail somehow.
Maybe you could check this.
Regards
Fabian