File upload doesn’t accept .xls

I had the same issue, this was the response that I received from support, and the problem went away after I added the option.

You are probably running into a WordPress security limitation where it limits what types of files can be uploaded. You can read more about expanding this to include additional formats here: https://www.wpbeginner.com/wp-tutorials/how-to-add-additional-file-types-to-be-uploaded-in-wordpress/

Also, you could try a plugin: https://www.ads-software.com/plugins/wp-extra-file-types (please note that I have not tested this plugin, so I can’t verify if it will work for you but it is available if that’s what you are more comfortable with).