A struct to hold a more structured representation of the Content-Disposition header.
This is provided mostly as a convenience since this metadata is usually
needed to interpret the message body properly.
The possible disposition types in a Content-Disposition header. A more
comprehensive list of IANA-recognized types can be found at
https://www.iana.org/assignments/cont-disp/cont-disp.xhtml. This library
only enumerates the types most commonly found in email messages, and
provides the Extension value for holding all other types.