Added caption extraction when send a msg with a file (#4499)

Closes #4499

telethon/client/messages.py

@@ -826,6 +826,8 @@ class MessageMethods:
                 await client.send_message(chat, 'Hi, future!', schedule=timedelta(minutes=5))
         """
         if file is not None:
+            if isinstance(message, types.Message):
+                message = message.message
             return await self.send_file(
                 entity, file, caption=message, reply_to=reply_to,
                 attributes=attributes, parse_mode=parse_mode,

tests/telethon/client/test_messages.py

@@ -1,8 +1,12 @@
 import inspect
+from unittest import mock
+from unittest.mock import MagicMock
 
 import pytest
 
 from telethon import TelegramClient
+from telethon.client import MessageMethods
+from telethon.tl.types import PeerChat, MessageMediaDocument, Message
 
 
 @pytest.mark.asyncio
@@ -38,3 +42,36 @@ async def test_send_message_with_file_forwards_args():
 
     client = MockedClient()
     assert (await client.send_message('a', file='b', **arguments)) == sentinel
+
+
+class TestMessageMethods:
+    @pytest.mark.asyncio
+    async def test_send_msg_and_file(self):
+        async def async_func(result): # AsyncMock was added only in 3.8
+            return result
+        msg_methods = MessageMethods()
+        expected_result = Message(
+            id=0, peer_id=PeerChat(chat_id=0), message='', date=None,
+        )
+        entity = 'test_entity'
+        message = Message(
+            id=1, peer_id=PeerChat(chat_id=0), message='expected_caption', date=None,
+        )
+        media_file = MessageMediaDocument()
+
+        with mock.patch.object(
+            target=MessageMethods, attribute='send_file',
+            new=MagicMock(return_value=async_func(expected_result)), create=True,
+        ) as mock_obj:
+            result = await msg_methods.send_message(
+                entity=entity, message=message, file=media_file,
+            )
+            mock_obj.assert_called_once_with(
+                entity, media_file, caption=message.message,
+                reply_to=None, attributes=None, parse_mode=(),
+                force_document=False, thumb=None, buttons=None,
+                clear_draft=False, silent=None, schedule=None,
+                supports_streaming=False, formatting_entities=None,
+                comment_to=None, background=None, nosound_video=None,
+            )
+            assert result == expected_result