Fix stripped image downloads (#1165)

telethon/client/downloads.py

@@ -344,7 +344,9 @@ class DownloadMethods(UserMethods):
             f = file
 
         try:
-            f.write(size.bytes)
+            f.write(utils.stripped_photo_to_jpg(size.bytes)
+                    if isinstance(size, types.PhotoStrippedSize)
+                    else size.bytes)
         finally:
             if isinstance(file, str):
                 f.close()