Add files pattern to ignore EXTRA_FILE checker errors

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Is your feature request related to a problem? Please describe.
The code signing feature is a valuable one. However, to increase performance of my personal server, I use the combination of brotli_static on; in nginx, and pre-generate all brotli files (using static-compress). Thus it creates a bunch of .br files in the directory.

Describe the solution you'd like
I'd like a setting to describe which files I know for sure are EXTRA_FILEs. Ideally, this would be a regexp, so that I could ignore all .br files.

Describe alternatives you've considered

• Disabling brotli_static on;. Files are recompressed on each new request. My server is not powerful, so I'd like to avoid that.
• Serve static files from outside code signing root, but it's a heavy gymnastic on each upgrade.

Additional context
None

Start with the linked code-signing documentation and locate the EXTRA_FILE checker in the Nextcloud server code. Determine how a configurable file pattern could be represented and applied safely, then verify that matching .br files are ignored while unrelated unexpected files remain reported. The payload names no implementation files or tests.