feat: implement file extension blacklist for upload security (#27540)

api/configs/feature/__init__.py

@@ -331,6 +331,31 @@ class FileUploadConfig(BaseSettings):
         default=10,
     )
 
+    inner_UPLOAD_FILE_EXTENSION_BLACKLIST: str = Field(
+        description=(
+            "Comma-separated list of file extensions that are blocked from upload. "
+            "Extensions should be lowercase without dots (e.g., 'exe,bat,sh,dll'). "
+            "Empty by default to allow all file types."
+        ),
+        validation_alias=AliasChoices("UPLOAD_FILE_EXTENSION_BLACKLIST"),
+        default="",
+    )
+
+    @computed_field  # type: ignore[misc]
+    @property
+    def UPLOAD_FILE_EXTENSION_BLACKLIST(self) -> set[str]:
+        """
+        Parse and return the blacklist as a set of lowercase extensions.
+        Returns an empty set if no blacklist is configured.
+        """
+        if not self.inner_UPLOAD_FILE_EXTENSION_BLACKLIST:
+            return set()
+        return {
+            ext.strip().lower().strip(".")
+            for ext in self.inner_UPLOAD_FILE_EXTENSION_BLACKLIST.split(",")
+            if ext.strip()
+        }
+
 
 class HttpConfig(BaseSettings):
     """