email: Accept inline body with disposition params

A mail client may indicate a header parameter (such as Novell Groupwise,
which may add something like

Content-Disposition: inline; modification-time: "Thu ..."

) Previously, the system would not consider the content to be a candidate
for the email body, because it has parameters to the content-disposition
header.

This patch will still consider the part to be a candidate for the body if
the mime types match and no filename is found in the content-disposition
parameters.