Use Dropbox and upload the file there; paste the URL of the file into the URL field in RTM; voila. Works very nicely.

that's a lot of extra steps. I think attachments need to be part of the task creation, or taken from the email.

Dropbox is nice,

but if the attachment if worked directly with RTM you can for example if you have email with attachment you can just forward it to your private email and add it as an Item in one of your list to fill it later for example.

As always, when I make my suggestions, I am not trying to suggest an idea is not without merit. I am suggesting it can be accomplished now with other, free tools. Given the pace of development at RTM, we have a choice of either suggesting things and getting frustrated when Bob & Co. never act on those suggestions, or we can build on the functionality that is available and build on that to deliver the tools we desire to manage our tasks.

I am sure attachments would be handy to a lot of people, but I also think that RTM likely has no interest in people uploading MB upon MB of files. Perhaps as a Pro-only feature?