Do not append file into FormData if already has !

Sometimes people may want to minimize the file size and will do some compress then set the new compressed file into the form.
Therefore when data already contains new file, do not append the original one.

Author: ablipan

SimpleAjaxUploader.js

@@ -1756,14 +1756,22 @@ ss.XhrUpload = {
 
         if ( opts.multipart === true ) {
             var formData = new FormData();
-
+            
+            var hasFile = false;
+            
             for ( var prop in params ) {
                 if ( params.hasOwnProperty( prop ) ) {
+                    if (prop === opts.name) {
+                        hasFile = true;
+                    }
                     formData.append( prop, params[prop] );
                 }
             }
-
-            formData.append( opts.name, fileObj.file );
+            
+            if (!hasFile) {
+                formData.append( opts.name, fileObj.file );
+            }
+            
             this.log( 'Commencing upload using multipart form' );
             xhr.send( formData );