perf(policy): make 'filetype' fallback optional, add 'empty' option

[linrongbin16]

Relate: #179 .

[codecov]

Codecov Report

Attention: 13 lines in your changes are missing coverage. Please review.

Comparison is base (591a727) 48.29% compared to head (37eb20a) 47.48%.

❗ Current head 37eb20a differs from pull request most recent head 5090ddd. Consider uploading reports for the commit 5090ddd to get more accurate results

Files	Patch %	Lines
lua/colorbox.lua	7.14%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #180      +/-   ##
==========================================
- Coverage   48.29%   47.48%   -0.81%     
==========================================
  Files           2        2              
  Lines         528      537       +9     
==========================================
  Hits          255      255              
- Misses        273      282       +9     

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