Add Canon Raw CR2 mimeType

[ireun]

mime package lacks entry for Canon Raw CR2 file format, this PR fixes that

source: https://github.com/lclevy/libcraw2/blob/master/docs/cr2_poster.svg

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[Xentraxx]

@ireun I feel like google doesn't want us to build a tool to escape google photos :P I ended up here with another issue with the mime package as well: https://github.com/dart-lang/tools/issues/2131