Fix utils._get_extension not working in pathlib objects

This was found while testing #1371.

tests/telethon/test_utils.py

@@ -1,3 +1,6 @@
+import io
+import pathlib
+
 from telethon import utils
 from telethon.tl.types import (
     MessageMediaGame, Game, PhotoEmpty
@@ -16,3 +19,37 @@ def test_game_input_media_memory_error():
     ))
     input_media = utils.get_input_media(media)
     bytes(input_media)  # <- shouldn't raise `MemoryError`
+
+
+def test_private_get_extension():
+    # Positive cases
+    png_header = bytes.fromhex('89 50 4e 47 0d 0a 1a 0a  00 00 00 0d 49 48 44 52')
+    png_buffer = io.BytesIO(png_header)
+
+    class CustomFd:
+        def __init__(self, name):
+            self.name = name
+
+    assert utils._get_extension('foo.bar.baz') == '.baz'
+    assert utils._get_extension(pathlib.Path('foo.bar.baz')) == '.baz'
+    assert utils._get_extension(png_header) == '.png'
+    assert utils._get_extension(png_buffer) == '.png'
+    assert utils._get_extension(png_buffer) == '.png'  # make sure it did seek back
+    assert utils._get_extension(CustomFd('foo.bar.baz')) == '.baz'
+
+    # Negative cases
+    null_header = bytes.fromhex('00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00')
+    null_buffer = io.BytesIO(null_header)
+
+    empty_header = bytes()
+    empty_buffer = io.BytesIO(empty_header)
+
+    assert utils._get_extension('foo') == ''
+    assert utils._get_extension(pathlib.Path('foo')) == ''
+    assert utils._get_extension(null_header) == ''
+    assert utils._get_extension(null_buffer) == ''
+    assert utils._get_extension(null_buffer) == ''  # make sure it did seek back
+    assert utils._get_extension(empty_header) == ''
+    assert utils._get_extension(empty_buffer) == ''
+    assert utils._get_extension(empty_buffer) == ''  # make sure it did seek back
+    assert utils._get_extension(CustomFd('foo')) == ''