A body made of multiple parts.
Usage: A multipart body acts as a container for other bodies. For instance,
the multipart body is commonly used to send a form with binary attachments in conjunction with
For simple key/value pairs, use URLEncodedBody instead.
Safety consideration: A random boundary is generated to separate parts. If the boundary was
to occur within a body part, it would mess up the whole body. In practice, the odds are extremely small though.
The pieces of body. The key in the map is used as name for the Content-Disposition header
of each part.
The exact multipart mime type as in multipart/subtype. Defaults to form-data.