Content filtering fails to collapse alternatives.

[Mark Sapiro]

Collapsing multipart/alternative parts to just the first alternative only works if the message itself is multipart/alternative, or the multipart/alternative part is at the first sub-level.

For example, given this structure:

multipart/mixed
| multipart/related
| | multipart/alternative
| | | text/plain
| | | text/html
| | image/jpeg
| application/msword

The multipart/alternative part will not be collapsed.