Fix path traversal when downloading documents

Closes #4713.

telethon/client/downloads.py

@@ -1054,8 +1054,11 @@ class DownloadMethods:
 
         if os.path.isdir(file) or not file:
             try:
+                isreserved = getattr(os.path, 'isreserved', lambda _: False)  # Python 3.13 and above
                 name = None if possible_names is None else next(
-                    x for x in possible_names if x
+                    x  # basename to prevent path traversal (#4713)
+                    for x in map(os.path.basename, possible_names)
+                    if x and not isreserved(x)
                 )
             except StopIteration:
                 name = None