[FIX] mail: convert attachments to binary as it is the expected format of message_post opw 604205

The double convertion (render_message and send_mail) is done to keep the API but should be changed in next version.
[FIX] mail: mail: convert attachments to binary as it is the expected format of message_post opw 604205

The double convertion (render_message and send_mail) is done to keep the API but should be changed in next version.