get the Writer's boundary
get the buffer of the Writer
finish the multipart message and writes the trailing boundary end line to the output.
get the Content-Type for an HTTP multipart/form-data with this Writer's Boundary.
set the boundary. setBoundary must be called before any parts are created.
boundary separator
write the mimeHeader data to the buffer
create form field, and then writes the given value
field name
value
multipart Writer interface