Ignore invalid supplied MIME types

[Gallaecio]

Based on a real-world scenario found as part of scrapy/scrapy#5204 (comment).

According to the standard, step 5 in https://mimesniff.spec.whatwg.org/#interpreting-the-resource-metadata:

If supplied-type is not a MIME type, the supplied MIME type is undefined.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (062c229) 100.00% compared to head (6054230) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #18   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         4           
  Lines          352       367   +15     
=========================================
+ Hits           352       367   +15     

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