Expose register method

[aoliva-sefas]

Allow library's user to dynamically register its own file type using register method.

This could be useful for custom file format that needs to be accepted in a NodeJS server (for instance)

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Latest commit: 59c1c1303df8a1592f585b026568e52945e8d8dc

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[LarsKoelpin]

Thanks for your contribution, I will have a look later!