"\cuteeditor_files\Configuration\Security", you can find 3 security policy files in this folder, which security policy file are you using? you can try to add "xls" and "xlsx" in all 3 security policy files. If it stil doesn't work, please check

<security name="DocumentFilters">   // When you click "Downloadable files"

<security name="ImageFilters">  // When you click "Insert Image" and "Image Gallery"

<security name="MediaFilters"> //When you click "Insert Media"

 

Which button did you click when you have this issue?

Keep us posted.

 

Regards,

Eric