Update UploadTask.java

Changes the content type to use `multipart/form-data` rather than `multipart/mixed` which some target servers do not know how to handle.

This makes it match what is sent on the iOS platform as well.

Author: Nick Vahalik

src/android/UploadTask.java

@@ -357,6 +357,7 @@ private Request createRequest() throws FileNotFoundException {
         }
 
         bodyBuilder.addFormDataPart(fileKey, filepath, fileRequestBody);
+        bodyBuilder.setType(MultipartBody.FORM);
 
         // Start build request
         String method = getInputData().getString(KEY_INPUT_HTTP_METHOD);