The value is a comma separated list of content types by priority, with the first being highest priority. The body part with the highest priority is selected as the main body part. According to the above rule, a "text/plain" part is selected as the main part over text/html or any other like text/xml etc.
The new implementation also drops alternative parts, or parts without a name. For example, if someone considers a text/xml part as the main part (say a SOAP message), any text/plain and/or text/html alternatives are dropped altogether. By setting the following attribute to true at the proxy service or mail transport listener level, one would be able to retain these as "unnamed-attachment-<n>" etc.