docs: make warning for security matter

Author: kenjis

user_guide_src/source/libraries/uploaded_files.rst

@@ -196,11 +196,12 @@ Other File Info
 
 **getClientExtension()**
 
-Returns the original file extension, based on the file name that was uploaded. This is NOT a trusted source. For a
-trusted version, use ``guessExtension()`` instead::
+Returns the original file extension, based on the file name that was uploaded::
 
     $ext = $file->getClientExtension();
 
+.. warning:: This is NOT a trusted source. For a trusted version, use ``guessExtension()`` instead.
+
 **getClientMimeType()**
 
 Returns the mime type (mime type) of the file as provided by the client. This is NOT a trusted value. For a trusted