When the attachment's suggested filename has spaces or other
undesirable characters, munpack replaces them with 'X'.
ThisXisXugly, and I'd prefer to let the user specify a replacement
character. Here is a patch that adds an option [-r character] so
that munpack -r- would produce This-is-better. The default is
still 'X', so as not to disrupt any expectations. The manual page
and usage string have been updated accordingly.