Fix RangeError on non-file drag/drop events (#1498)

ConversationView responds to drag/drop events by forwarding them to its file
input. The file input stops propagation and handles the event only if the data
transfer is type file. This means that any other data type (text, img, etc...)
causes an recursive loop of event propagation, eventually resulting in logging a
"RangeError: Maximum call stack size exceeded".

Fix by only forwarding files to the file input.

// FREEBIE

js/views/conversation_view.js

@@ -208,6 +208,9 @@
             'dragleave': 'sendToFileInput'
         },
         sendToFileInput: function(e) {
+            if (e.originalEvent.dataTransfer.types[0] != 'Files') {
+                return;
+            }
             this.fileInput.$el.trigger(e);
         },