Erin contacted The Guru with the below question. I tried doing a Google Search and checking the mozillaZine forums without much luck so may be someone else here can give us some advice:
Since we upgraded to Thunderbird version 18.104.22.168 it seems that not all HTML attachments are treated the same (when sent). Those with ^M are sent as “quoted-printable” text (results in a problem) and the same file without ^M is send encoded base64 (producing desired results). Is there a way to force Thunderbird to treat them all the same? Thanks.