Attachment encoding options

[MisterToolbox]

Attachments are by default, Base64 encoded. It'd be nice to have that be optional, either in a global Mailman config setting, or maybe per attachment by passing a tuple with the path and options to the inline function.

[skosch]

Are you suggesting 8-bit encoding? If so, can you provide a use case? Frankly, this hasn't come up as a priority, so I don't foresee this being added in the near future, but if you can offer a PR, go ahead.