Prevent default action only when dragging/dropping files - Fixes #1897.

Thanks to Ville Lautanala (lautis) for his contribution.

Author: blueimp

js/jquery.fileupload.js

@@ -1,5 +1,5 @@
 /*
- * jQuery File Upload Plugin 5.19.4
+ * jQuery File Upload Plugin 5.19.5
  * https://github.com/blueimp/jQuery-File-Upload
  *
  * Copyright 2010, Sebastian Tschan
@@ -952,10 +952,12 @@
         },
 
         _onDrop: function (e) {
-            e.preventDefault();
             var that = this,
                 dataTransfer = e.dataTransfer = e.originalEvent.dataTransfer,
                 data = {};
+            if (dataTransfer && dataTransfer.files && dataTransfer.files.length) {
+                e.preventDefault();
+            }
             this._getDroppedFiles(dataTransfer).always(function (files) {
                 data.files = files;
                 if (that._trigger('drop', e, data) !== false) {
@@ -969,10 +971,10 @@
             if (this._trigger('dragover', e) === false) {
                 return false;
             }
-            if (dataTransfer) {
+            if (dataTransfer && dataTransfer.files && dataTransfer.files.length) {
                 dataTransfer.dropEffect = 'copy';
+                e.preventDefault();
             }
-            e.preventDefault();
         },
 
         _initEventHandlers: function () {