|Policies concerning the content of list traffic.
Content filtering works like this: when a message is
received by the list and you have enabled content filtering, the
individual attachments are first compared to the
types. If the attachment type matches an entry in the filter
types, it is discarded.
Then, if there are pass types
defined, any attachment type that does not match a
pass type is also discarded. If there are no pass types defined,
this check is skipped.
After this initial filtering, any multipart
attachments that are empty are removed. If the outer message is
left empty after this filtering, then the whole message is
discarded. Then, each multipart/alternative section will
be replaced by just the first alternative that is non-empty after
Finally, any text/html parts that are left in the
message may be converted to text/plain if
convert_html_to_plaintext is enabled and the site is
configured to allow these conversions.