So, just out of curiosity, would you be willing to try an experiment and use an Alternate Entry Point to attach your attachment handler? I've never tried to do it the way you suggest, and for the application I have developed, this has always worked.
That is, with a few exceptions - Sprint's version of device OS 4.3.X does not work with email attachments very well. Our testing failed every single time, and RIM admitted that Sprint had accepted a version with a bug in the email attachment handling mechanism. Luckily no other service provider is using 4.3.X. However, early releases of OS 4.5 also contain the same bug, and several were accepted by Rogers Wireless and Vodafone Germany. There may be one or two others I've left off the list. So if you're using a 4.3 simulator and/or a 4.3-based device, email attachment handling may not work correctly (I think I was seeing failure in both the simulators and real devices).
Karl G. Kowalski
Owns a RAZR
Develops for BlackBerry
So next phone will be........an iPhone 3G!