I can attach a picture file to an email.
The picture file is "attachable"
, is there a single word adjective that describes quality of the email that it can be attached to?
If the picture can be attached to the email, the picture is attachable and the email is attachment-capable.
How about attachment-bearing?
It doesn't necessarily roll off the tongue, but that's what I'd probably use.
EDIT: Even before @TrevorD pointed it out, I realized my response wrongly addresses an email that has an attachment rather than one capable of having one. Mea culpa.
Could you say - this software is upload-friendly, attachment-friendly or attachment-enabled?