plugin XEP-0363: fixes a forgotten attribute update

src/plugins/plugin_xep_0363.py

@@ -234,7 +234,7 @@ class XEP_0363(object):
         @param content_type(unicode, None): MIME type of the content
             empty string or None to guess automatically
         """
-        filename.replace('/', '_')
+        filename = filename.replace('/', '_')
         client = self.host.getClient(profile_key)
         return  self.getSlot(client, filename, size, content_type or None, upload_jid or None)