fix(api): force download for HTML previews (#30090)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

api/controllers/common/file_response.py

@@ -0,0 +1,57 @@
+import os
+from email.message import Message
+from urllib.parse import quote
+
+from flask import Response
+
+HTML_MIME_TYPES = frozenset({"text/html", "application/xhtml+xml"})
+HTML_EXTENSIONS = frozenset({"html", "htm"})
+
+
+def _normalize_mime_type(mime_type: str | None) -> str:
+    if not mime_type:
+        return ""
+    message = Message()
+    message["Content-Type"] = mime_type
+    return message.get_content_type().strip().lower()
+
+
+def _is_html_extension(extension: str | None) -> bool:
+    if not extension:
+        return False
+    return extension.lstrip(".").lower() in HTML_EXTENSIONS
+
+
+def is_html_content(mime_type: str | None, filename: str | None, extension: str | None = None) -> bool:
+    normalized_mime_type = _normalize_mime_type(mime_type)
+    if normalized_mime_type in HTML_MIME_TYPES:
+        return True
+
+    if _is_html_extension(extension):
+        return True
+
+    if filename:
+        return _is_html_extension(os.path.splitext(filename)[1])
+
+    return False
+
+
+def enforce_download_for_html(
+    response: Response,
+    *,
+    mime_type: str | None,
+    filename: str | None,
+    extension: str | None = None,
+) -> bool:
+    if not is_html_content(mime_type, filename, extension):
+        return False
+
+    if filename:
+        encoded_filename = quote(filename)
+        response.headers["Content-Disposition"] = f"attachment; filename*=UTF-8''{encoded_filename}"
+    else:
+        response.headers["Content-Disposition"] = "attachment"
+
+    response.headers["Content-Type"] = "application/octet-stream"
+    response.headers["X-Content-Type-Options"] = "nosniff"
+    return True