Currently, there are two types of upload handlers available: UploadHandlerRaw and UploadHandlerFile.
UploadHandlerRaw class accepts a data buffer at construction time. When this buffer is an array of bytes, it’s copied internally into native code memory. UnityWebRequest system uses this buffer when the remote server is ready to receive the request body data. When the buffer is provided as a NativeArray, no copying is performed.
UploadHandlerFile allows you to send the contents of a file as the request body. Using this handler, you can send a large file to a server without using a lot of memory. As the handler reads data from the file and then sends it, only a small fraction of the file is kept in memory at any given time.
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.
UnityWebRequest has a property disposeUploadHandlerOnDispose, which defaults to true. If this property is true, when UnityWebRequest object is disposed, Dispose() will also be called on attached upload handler rendering it useless. If you keep a reference to upload handler longer than the reference to UnityWebRequest, you should set disposeUploadHandlerOnDispose to false.
byte[] payload = new byte[1024];
// ... fill payload with data ...
UnityWebRequest wr = new UnityWebRequest("https://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;