Honestly, we did not do any work or testing to make extensions available for mobile so I am not surprised to see an extension not working there.
I tried using your API but not sure it doesn't give all the fields from the data source in the configure window when I put an extension on a dashboard.
The extension will only show fields that match the data type of the parameter you chose to use. Make sure that the field and parameter have the same data type and try again. If it still doesn't work, feel free to open up a new question here and tag me.