plugins XEP-0096, XEP-0260, file: sendFile method is managed by file plugin, which choose the best available method + progress_id fix

src/core/sat_main.py

@@ -839,7 +839,7 @@ class SAT(service.Service):
         try:
             del client._progress_cb[progress_id]
         except KeyError:
-            log.error(_("Trying to remove an unknow progress callback"))
+            log.error(_(u"Trying to remove an unknow progress callback"))
 
     def _progressGet(self, progress_id, profile):
         data = self.progressGet(progress_id, profile)