Feature/improve file extensions

[renefloor]

✨ What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Improve parsing of MIME content-type.

⤵️ What is the current behavior?

Currently it just takes the subtype of the content-type as fileExtension. This works fine for types as image/png, but fails to differentiate between video/mpeg and video/mpeg. Content-types as application/vnd.openxmlformats-officedocument.presentationml.presentation just get really weird extensions.

🆕 What is the new behavior (if this is a feature change)?

It checks the most common content types from mozilla and parses them. If it is not in the list it falls back on previous behaviour.

💥 Does this PR introduce a breaking change?

No

🐛 Recommendations for testing

The current tests are still successful.

📝 Links to relevant issues/docs

🤔 Checklist before submitting

• All projects build
• Follows style guide lines (code style guide)
• Relevant documentation was updated
• Rebased onto current develop

[codecov]

Codecov Report

Merging #150 into develop will increase coverage by 0.2%.
The diff coverage is 83.33%.

@@            Coverage Diff             @@
##           develop     #150     +/-   ##
==========================================
+ Coverage     38.3%   38.51%   +0.2%     
==========================================
  Files            7        8      +1     
  Lines          295      296      +1     
==========================================
+ Hits           113      114      +1     
  Misses         182      182

Impacted Files	Coverage Δ
lib/src/web/web_helper.dart	0% <ø> (ø)
lib/src/cache_manager.dart	0% <ø> (ø)	⬆️
lib/src/web/file_fetcher.dart	87.87% <100%> (ø)
lib/src/web/mime_converter.dart	66.66% <66.66%> (ø)

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