Allowed extensions not working

For an application the excel importer is giving some issues, to make a quick fix for this we want to disable uploading any files other than xls. When I fill in the allowed extensions for the filemanager and run it locally it works fine, I can only upload .xls files. Once deployed to the server however, all files are allowed again. The server is running on windows on-premise. Any idea what could be causing this?
1 answers

Due to a special design in the server the static pages had to be copied to the IIS server (which is on another server). After coping this, all work fine now. Thanks all for helping!