ignorePattern doesn't seem to be working

Howdy 👋

Just trying to stop this gzipping my sourcemaps and I've been trying to add an ignorePattern that doesn't seem to be working 😕

I've tried `ignorePattern: '**.map'` and `ignorePattern: '*.map'` but they both still gzipped my sourcemaps. I even tried `ignorePattern: '\*\*/\*.map',` which I lifted from the default filePattern 😂

I ended up working around this by just using filePattern with `map` removed as an option, but I figured I should open an issue to see if there is anything I can do to help. I would love to add a failing test if someone could at least point me in the right direction.

Start by tracing how the plugin reads and applies ignorePattern, comparing it with the working filePattern option. Reproduce the sourcemap case using the reported patterns and add a failing test if the repository has an existing test entry point; done means matching .map files are left uncompressed while other eligible files are still gzipped.

Written by the indexing model from the issue text.