Bug: 14329 Ensure the file input node is clear.

If removing a attachment, if we don't manually clear the
input node on some browsers, it will be impossible to attach
the same file again since the browser thinks it is already
attached.

imp/js/compose-dimp.js

@@ -885,6 +885,8 @@ var DimpCompose = {
                 afterFinish: function() {
                     this.removeAttachRow(elt);
                     this.initAttachList();
+                    $('upload').removeAttribute('value');
+                    $('upload').replace($('upload').cloneNode(true));
                 }.bind(this),
                 duration: 0.4
             });