While I know what you mean, it's important to realize that the file still needs to get uploaded to the server, since you don't have filesystem access with anything client side that you're likely to be using.
It will be uploaded to a temporary location and will be accessible using an array, which I think is in the $HTTP_POST_FILES scope. Then you'll have to send the mail as multi-part using that attachment. I don't have code handy, but I'll post back if I can track it down. Here are some good places to get started with your research in the meantime.
As you probably know, there are potential risks involved with letting people upload any sort of file through a web-accessible form, that will then be forwarded to your email. Especailly if you're using an email client with html support or preview enabled...which you should consider not doing.