Indeed, the application doesn't have support for the files you try to load. I have tested myself with both XLSX and XLSM and it appears that you can load only the XLS files. As you can see in the image below.
I've copied other XLSM files in the same location with the XLS, but they are not seen by the app which means they're not supported. You will need an update to the MS Excel File Properties Changer in order to use the XLSM, but unfortunately, there isn't one available.
Try converting to XLS files and then load the files.