According to RFC1521, the type and subtype in the Content-Type header should be matched as case-insensitive. While most clients use all lower case, some, such as Pegasus Mail, do not. This prevents proper inclusion of the attachment (+) flag when displaying the message headers for email generated by such clients. Using strtolower during the check for 'multipart' and 'mixed' in functions/mailbox_display.php fixes the problem in the current stable version. While I'm not using the development version, the code there also appears case sensitive and should probably be modified as well.
If you have any questions, you can email me at firstname.lastname@example.org.