Mention WAGTAILDOCS_EXTENSIONS in 'integrating into Django' docs

docs/advanced_topics/add_to_django_project.md

@@ -339,6 +339,12 @@ WAGTAIL_SITE_NAME = 'My Project'
 # Wagtail email notification format
 # WAGTAILADMIN_NOTIFICATION_USE_HTML = True
 
+# Allowed file extensions for documents in the document library.
+# This can be omitted to allow all files, but note that this may present a security risk
+# if untrusted users are allowed to upload files -
+# see https://docs.wagtail.org/en/stable/advanced_topics/deploying.html#user-uploaded-files
+WAGTAILDOCS_EXTENSIONS = ['7z', 'bz2', 'csv', 'docx', 'gz', 'key', 'odt', 'pdf', 'pptx', 'rar', 'rtf', 'tar', 'txt', 'xlsx', 'zip']
+
 # Reverse the default case-sensitive handling of tags
 TAGGIT_CASE_INSENSITIVE = True
 ```

docs/getting_started/integrating_into_django.md

@@ -64,6 +64,12 @@ WAGTAILADMIN_BASE_URL = 'http://example.com'
 
 If this setting is not present, Wagtail will fall back to `request.site.root_url` or to the hostname of the request. Although this setting is not strictly required, it is highly recommended because leaving it out may produce unusable URLs in notification emails.
 
+Add a `WAGTAILDOCS_EXTENSIONS` setting to specify the file types that Wagtail will allow to be uploaded as documents. This can be omitted to allow all file types, but this may present a security risk if untrusted users are allowed to upload documents - see [](user_uploaded_files).
+
+```python
+WAGTAILDOCS_EXTENSIONS = ['7z', 'bz2', 'csv', 'docx', 'gz', 'key', 'odt', 'pdf', 'pptx', 'rar', 'rtf', 'tar', 'txt', 'xlsx', 'zip']
+```
+
 Various other settings are available to configure Wagtail's behaviour - see [Settings](/reference/settings).
 
 ## URL configuration