Add content type-to-extension conversion

[tmannherz]

Currently, the package only does file-to-content type conversion via the lookupMimeType function. As the extension maps already exist, it'd be easy to add both sides of the lookup, something like:

print(lookupExtension('text/html'));  // html
print(lookupExtension('application/pdf'));  // pdf

[kevmoo]

Pull requests welcome!

[d-silveira]

doesn't these reference commits close this issue?
@tmannherz did you request a pull?

[lifenautjoe]

Any clue when this might be released? 🙏

[ZuSe]

Pull request needs to be merged first
#15

[mx1up]

pull request #15 is closed, but without comment. Does anyone know the reason? Otherwise, I'd like to make a new PR with similar implementation

[kevmoo]

It was abandoned by the author. If someone else wants that functionality, starting with a new PR would be great!

[tmannherz]

Sorry, the related PR lingered for so long awaiting approval (~2 years) that I can't support it and don't do any active Dart development, so I closed it. Feel free to copy and take it over.