Add MimeType for .VSDX , .RAW, .MSG

[d3f4u1t3d]

I have added support for file extension ".vsdx",".msg" and ".raw" .

I have modified the code to check based on extensions.

Sorry for poor English.

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[mosuem]

As this repo has moved to https://github.com/dart-lang/tools/tree/main/pkgs/mime, please refile the PR there. Thanks!