mark application/vnd.pmtiles as not compressible

[bdon]

I'm the maintainer of a JavaScript implementation of the pmtiles specification.

Recently storing.pmtiles on GitHub Pages and accessing them via HTTP Range Requests stopped working only in Firefox, because Firefox was accepting gzip content and GitHub Pages was serving a compressed response, leading to unreadable data. Full description here: protomaps/PMTiles#584

According to GH support, Pages decides to compress responses based on MIME types in this DB. Renaming the .pmtiles file to .pmtiles.gz is a workaround, but does not fix existing Pages sites.

I would like to add a custom entry to src/custom-types.json defining .pmtiles as not compressible.