Save method will save attachment to external file on
disk, as specified in Filename
parameter. Filename is optional, and if it's not
specified, default Filename will be used as specified by
the sender.
Depending on Encoding
property, wodPop3 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.