Description of problem:

Thunderbird doesn't seem to offer an option to chose the type of encoding attachments.

E.g. thunderbird seems always base64 encode '*diff' attachment.

This practice is frowned upon in developer mailing lists, because it renders reading *.diff's hard.

Version-Release number of selected component (if applicable):
thunderbird-3.0-2.6.b3.fc11.x86_64

How reproducible:
Always

Steps to Reproduce:
1. Attach a *.diff to a mail
2. send it to yourself.
3. Check the mail's source
  
Actual results:

Sent mail contains a base64 encoded attachment:
...
Content-Type: text/plain;
 name="foo.diff"
Content-Transfer-Encoding: base64
Content-Disposition: attachment;


Expected results:
User to select type of attachment.

Additional info:
base64 encoded attachments are frowned upon on all gnu lists.

Provided the fact gnu lists are one of my primary use cases, this defect in thunderbird forces me to stop using thunderbird.