Been mentioned before, but the way you prevent attachments is by having
a size limit on all posts -- pick something between 100 and 250
depending on the type of list you have.  Most people use 100; I use 250
because sometimes we forward important material that needs to go out in
a longer form; 500 lines cut up into two posts is easier to handle than
500 lines chopped up into five posts!  And a 250 line limit will stop
the attachments -- which do indeed blow up the digest.

The command to put in your header is:
* Sizelim= 100

or 200, or 250, or whatever.

Mary