[mms] Fix regression in outputting Content-Disposition 'size' parameter if no other Content-Disposition parameter is present.

Partial revert of 9937e2d8c28655271243c0be07ac6763621ac9f6

That commit incorrectly ignored the 'size' parameter when determining
whether to output Content-Disposition header.

Instead... there is little need to add size information when using
parseMessage().  In fact, it is much more efficient to determine size
information by analyzing length of the temporary stream, rather than
determining size by using str_replace() on a string.