exo-compose-mail-1 sends URIs with spaces encoded as %20, accents encoded, etc which thunderbird/icedove fail to decrypt, thus sending the mail fails because it did not find the file to be attached.
Is this really and exo problem; i mean, %20 is kinda normal in the web world...
*** Bug 8748 has been marked as a duplicate of this bug. ***
Seems fixed now, I can "send to mail recipient" a file with space without problem.