One can send messages to an AT&T customer via [10-digit-cell-numer]@txt.att.net from their email client. I tried to send a file to my iPhone using this method (specifically an audio file) to no avail. The message came through but the file attachment was not present.
It seems you can send MMS messages to @mms.att.net
That was easy.
Sending file via email to MMS for ATT