Well I tried attaching an .xlsx file in one email, and an .xls in a different email (both sending/receiving from Thunderbird) and both seemed to send fine for me. The Content-Types are as expected:
Content-Type: multipart/mixed;
Content-Type: application/octet-stream;
name="****.xls"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename="****.xls"
boundary="------------000904040502050606020608"
Although, by happenstance I uninstalled Excel on my computer just yesterday as a matter of fact, which is probably why it's being treated as a generic file attachment for me.