: multipart?
    ᡼뤬 MIME ޥѡȤΤȤ
    Υ᥽åɤ Content-Type إåƤǿȽǤޤ

: multipart? -> bool
    Content-Type  MIME ޥѡȥ᡼Ǥ뤳Ȥ
    Ƥʤп

