You need to add the given code in your wp-content/plugins/wplms-customizer/wplms-customizer.php file :
add_post_type_support(‘unit’,’thumbnail’);
Then goto to edit unit page from backend and click on add media button from the dropdown select uploaded to this page only.
Then click on upload media tab and upload a file (not image).You will see that file is selected in the popup then simply close the popup and update the unit and check.
