diff --git a/telethon/telegram_client.py b/telethon/telegram_client.py
index 73a5b66c..ad917f71 100644
--- a/telethon/telegram_client.py
+++ b/telethon/telegram_client.py
@@ -67,7 +67,7 @@ from .tl.types import (
     PeerUser, InputPeerUser, InputPeerChat, InputPeerChannel, MessageEmpty,
     ChatInvite, ChatInviteAlready, PeerChannel, Photo, InputPeerSelf,
     InputSingleMedia, InputMediaPhoto, InputPhoto, InputFile, InputFileBig,
-    InputDocument, InputMediaDocument
+    InputDocument, InputMediaDocument, Document
 )
 from .tl.types.messages import DialogsSlice
 from .extensions import markdown, html
@@ -1308,7 +1308,7 @@ class TelegramClient(TelegramBareClient):
             return self._download_photo(
                 media, file, date, progress_callback
             )
-        elif isinstance(media, MessageMediaDocument):
+        elif isinstance(media, (MessageMediaDocument, Document)):
             return self._download_document(
                 media, file, date, progress_callback
             )
@@ -1319,7 +1319,6 @@ class TelegramClient(TelegramBareClient):
 
     def _download_photo(self, photo, file, date, progress_callback):
         """Specialized version of .download_media() for photos"""
-
         # Determine the photo and its largest size
         if isinstance(photo, MessageMediaPhoto):
             photo = photo.photo
@@ -1345,9 +1344,13 @@ class TelegramClient(TelegramBareClient):
         )
         return file
 
-    def _download_document(self, mm_doc, file, date, progress_callback):
+    def _download_document(self, document, file, date, progress_callback):
         """Specialized version of .download_media() for documents"""
-        document = mm_doc.document
+        if isinstance(document, MessageMediaDocument):
+            document = document.document
+        if not isinstance(document, Document):
+            return
+
         file_size = document.size
 
         possible_names = []
@@ -1361,7 +1364,7 @@ class TelegramClient(TelegramBareClient):
                 ))
 
         file = self._get_proper_filename(
-            file, 'document', utils.get_extension(mm_doc),
+            file, 'document', utils.get_extension(document),
             date=date, possible_names=possible_names
         )
 
diff --git a/telethon/utils.py b/telethon/utils.py
index 16257be2..3e310d3d 100644
--- a/telethon/utils.py
+++ b/telethon/utils.py
@@ -61,13 +61,13 @@ def get_extension(media):
 
     # Documents will come with a mime type
     if isinstance(media, MessageMediaDocument):
-        if isinstance(media.document, Document):
-            if media.document.mime_type == 'application/octet-stream':
-                # Octet stream are just bytes, which have no default extension
-                return ''
-            else:
-                extension = guess_extension(media.document.mime_type)
-                return extension if extension else ''
+        media = media.document
+    if isinstance(media, Document):
+        if media.mime_type == 'application/octet-stream':
+            # Octet stream are just bytes, which have no default extension
+            return ''
+        else:
+            return guess_extension(media.mime_type) or ''
 
     return ''