Added x-aac for #74

[renefloor]

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

Add x-aac mimeType

⤵️ What is the current behavior?

Makes .x-aac file

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

Makes .aac file

💥 Does this PR introduce a breaking change?

No

🐛 Recommendations for testing

📝 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 #172 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #172   +/-   ##
========================================
  Coverage    79.89%   79.89%           
========================================
  Files           12       12           
  Lines          388      388           
========================================
  Hits           310      310           
  Misses          78       78           

Impacted Files	Coverage Δ
lib/src/web/mime_converter.dart	100.00% <ø> (ø)

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