code: public function asMime()
      {
          $part = 'Content-Disposition: form-data; ';
          $part .= 'name="' . $this->key . '"; ';
          $part .= 'filename="' . $this->filename . '"';
          $part .= "\r\nContent-Type: " . $this->deduceMimeType();
          $part .= "\r\n\r\n" . $this->content;
  
          return $part;
      }
docs: The MIME part as a string.
  
  @return string        MIME part encoding.