Fix file input bug introduced in 28c602a

Fix bug where clicking the attachment button to select a file would
immediately send any text already entered into the message input.

// FREEBIE

js/views/file_input_view.js

@@ -38,7 +38,8 @@
             'dragleave': 'hideArea'
         },
 
-        open: function() {
+        open: function(e) {
+            e.preventDefault();
             // hack
             if (this.window && this.window.chrome && this.window.chrome.fileSystem) {
                 this.window.chrome.fileSystem.chooseEntry({type: 'openFile'}, function(entry) {