Allow for webp tiles sources

[jo-chemla]

Hi there, following #60, there is a bug with webp tile sources that cannot be validated by NextGIS QMS? The source loads correctly on the leaflet map canvas, but cannot be made public with error Service is failing validation and can't be made public. Edit it to fix errors or let us know. + errors comes with text service response is not image.

https://qms.nextgis.com/geoservices/6088/