Currently, only one type of upload handler is available: UploadHandlerRaw. This class accepts a data buffer at construction time. This buffer is copied internally into native code memory and then used by the UnityWebRequest system when the remote server is ready to accept body data.
Upload Handlers also accept a Content Type string. This string is used for the value of the UnityWebRequest’s Content-Type header if you set no Content-Type header on the UnityWebRequest itself. If you manually set a Content-Type header on the UnityWebRequest object, the Content-Type on the Upload Handler object is ignored.
If you do not set a Content-Type on either the UnityWebRequest or the UploadHandler, the system defaults to setting a Content-Type of application/octet-stream.
byte[] payload = new byte[1024];
// ... fill payload with data ...
UnityWebRequest wr = new UnityWebRequest("http://www.mysite.com/data-upload");
UploadHandler uploader = new UploadHandlerRaw(payload);
// Sends header: "Content-Type: custom/content-type";
uploader.contentType = "custom/content-type";
wr.uploadHandler = uploader;