Error on uploading files to webform after update to localgov_base_preprocess_file_link

Raised on merge Tuesday. 

After https://github.com/localgovdrupal/localgov_base/commit/bc04fcef58d15795c5023ec140446ff45fa6646c webform file uploads are affected because they are not media items.

Add relevant checks to the function.

@andybroomfield had a look at some code and will look at setting up a PR to test.