Signals that this [UploadHandler] is no longer being used, and should clean up any resources it is using.
This method must be called once you have finished using an [UploadHandler] object.
For convenience, [UnityWebRequest] exposes the [UnityWebRequest.disposeUploadHandlerOnDispose] property, which will automatically call this method when [UnityWebRequest.Dispose] is called.
If you elect not to use [UnityWebRequest.disposeUploadHandlerOnDispose] (by setting it to false), then you must call Dispose on the [UploadHandler] yourself. This should only be in rare cases, such as when you wish to use the [UploadHandler] to transmit the same data multiple times.