property key in post multipart content (new in 1.54)
Optional. A property list defining additional header fields at the top level of the message. Alternatively, top level header fields can be given with the URL being posted to by specifying with headers headerFields after the URL. If a "MIME-Version" property is not given in TopHeader, it will automatically be included with a value of "1.0". If a "Content-Type" property is not included, one will automatically be generated which specifies a content type of "multipart/form-data" and a boundary string as defined by the MultipartBoundary property (or generated automatically it none is specified).