Save method will save attachment to external file on
disk, as specified in Filename
parameter. Filename is optional, and if not specified
default Filename will be used, as specified by the
sender.
Depending on Encoding
property, wodMailbox will decide which decoding algorithm
should be used when saving file, so that attachment is
properly decoded and saved (as binary raw data) to
specified filename.
If Filename exists, it will be deleted prior to
saving.