Missing .msg extension in FileExtensionContentTypeProvider

[fmflurry]

Describe the bug

Using FileExtensionContentTypeProvider TryGetContentType method with .msg file extension will not return application/vnd.ms-outlook.

We are using it to set Content-Type response header based on the file extension our user is trying to download. So far it’s working great but we had an issue where .msg files couldn’t be downloaded, and we found out in source code it is not supported.
https://github.com/dotnet/aspnetcore/blob/main/src/Middleware/StaticFiles/src/FileExtensionContentTypeProvider.cs

As a workaround we currently add .msg entry into Mappings but we believe that would be nice to support such a common extension.

[Tratcher]

FYI:

aspnetcore/src/Middleware/StaticFiles/src/FileExtensionContentTypeProvider.cs

Lines 17 to 18 in f1b907b

	// - We only plan to add new entries where we expect them to be applicable to a majority of developers such as being
	// used in the project templates.

[fmflurry]

I'm aware of that comment, in my honest opinion .msg should be supported as outlook format is widely used in companies.
The change is trivial and could avoid bugs / headaches in the future.

Shall I open a PR or you're not planning to add a new entry for .msg ?

[adityamandaleeka]

Triage: we don't think this meets the bar for inclusion per the referenced comment.

Since people can modify this list themselves, we don't want to update the defaults for this.