Safe file extension in uppercase refused

In our organisation the Document Management System produces file extensions in uppercase (MyLetter.DOC)
CiviCRM does not reckognise the capitalized extensions and changes the filename to MyLetter_DOC.unknown.

I have overriden CRM_Utils_Files::isExtensionSafe()
line 259

Code:
// wrap $ext into strtolower to allow for Uppercase extensions
return isset( $extensions[strtolower($ext)] ) ? true : false;