loadplugin Mail::SpamAssassin::Plugin::MIMEHeader mimeheader NAME_OF_RULE Content-Id =~ /foo/
Note that in a message of multiple parts, each header will be checked against the pattern separately. In other words, if multiple parts have a 'Content-Type' header, each header's value will be tested individually as a separate string.
Header names are considered case-insensitive.
The header values are normally cleaned up a little; for example, whitespace around the newline character in ``folded'' headers will be replaced with a single space. Append ":raw" to the header name to retrieve the raw, undecoded value, including pristine whitespace, instead.