Yarn provides a facility that should be adequate for your needs.
Compose your outgoing message with your editor just like you always
do. When you exit from your editor back to Yarn, Yarn gives you
a pop-up menu. Choose "Attach File". Yarn will ask you for the
file name (which you would normally type in complete with path).
When done, Yarn gives you the same pop-up menu again. (If need be,
you could choose "Attach File" again.) When you have no more
files to be attached to this message, choose "Send" in the menu.
The only drawback is that _every_ file you so attach will be
BASE64 encoded. For that reason, I sometimes "attach" text files
by doing a "get" when I am within my editor -- i.e., the file I
am "including" becomes part of the text of the message I write.
(This saves the expansion caused by BASE64 encoding.)
The composite built by Yarn with "Attach File" will be a MIME
multi-part message -- i.e., it will be "posted" properly wherever
the original message goes (because the attached file becomes just
another body part _within_ the whole of what you created).