To add this widget to your Enketo Express installation see this guidance.
Works on all regular "geo*" questions with the "mbtiles" appearance.
An example is available in xlsform_mbtiles_example.xlsx XLSForm.
This widget allow to add a mbtiles files on a geo* question.
Mbtiles files are stored in the indexedDB for the offline mode.
And the mbtile is automaticly load when you refresh the page.