Why can't I run Mergo on my MS Excel files?

I'm trying to use a MS Excel file saved in my Google Drive, but Mergo is telling me that it cannot work on Excel files directly. Is this a bug, and how can I fix it?

Since Mergo is an add-on for Google, it cannot work directly with non-Google-native formats - including MS Excel files (.xls or .xlsx formats). 

To work with Excel files in Mergo, please use any of the following methods: