If the attachment filename contains CJK chars, the sendto action will fail. Reporting message: An error occurred while creating a message compose window. Please try again. I am using icedove. The customized action icedove -compose 'attachment=file://%F' works. Example filename : "你好.txt"
I could not reproduce this issue with Thunar 1.6.3 (Xfce 4.10), but I am using Thunderbird as a default mail client. A Google search on the error message shows it is related to icedove, so the bug is most likely not in Thunar and this report should be sent to the icedove developers.
Even with thunderbird, it will not always work correctly. The filename needs to be URI-encoded, e.g.: python -c "import urllib; print 'file://' + urllib.quote('''$@''')" I am closing this bug because of its age. Reopen if you deem necessary.