markdown: Use re-exported guess_type to ensure adding EXTRA_MIME_TYPES.

Signed-off-by: Anders Kaseorg <anders@zulip.com>

tools/semgrep-py.yml

@@ -52,6 +52,19 @@ rules:
     languages: [python]
     severity: ERROR
 
+  - id: use-wrapped-mimetypes
+    pattern-either:
+      - pattern: mimetypes.guess_type
+      - pattern: mimetypes.guess_file_type
+      - pattern: mimetypes.guess_all_extensions
+      - pattern: mimetypes.guess_extension
+    message: Use zerver.lib.mime_types (to ensure our EXTRA_MIME_TYPES are added)
+    languages: [python]
+    severity: ERROR
+    paths:
+      exclude:
+        - zerver/lib/mime_types.py
+
   - id: dont-import-models-in-migrations
     patterns:
       - pattern-not: from zerver.lib.migrate import $X

zerver/lib/markdown/__init__.py

@@ -1,7 +1,6 @@
 # Zulip's main Markdown implementation.  See docs/subsystems/markdown.md for
 # detailed documentation on our Markdown syntax.
 import logging
-import mimetypes
 import re
 import time
 from collections import deque
@@ -1032,7 +1031,7 @@ class InlineInterestingLinkProcessor(markdown.treeprocessors.Treeprocessor):
         if not self.zmd.image_preview_enabled:
             return False
 
-        url_type = mimetypes.guess_type(url)[0]
+        url_type = guess_type(url)[0]
         # Support only video formats (containers) that are supported cross-browser and cross-device. As per
         # https://developer.mozilla.org/en-US/docs/Web/Media/Formats/Containers#index_of_media_container_formats_file_types
         # MP4 and WebM are the only formats that are widely supported.