Review FileExtensionContentTypeProvider and document the criteria for being included

[Tratcher]

FileExtensionContentTypeProvider contains the file extension to content-type mappings used by the StaticFilesMiddleware. Only extensions listed will be served by default. The original list was copied from IIS but we've made some additions.

• Review the APIs and make sure it's easy enough for people to add and remove entries.
• Document the criteria for including an entry in the default list.
• Review the current entries to make sure they meet the criteria.

@DamianEdwards

[DamianEdwards]

As part of the first item, I'd like to see what it would like to have an app set up to read a config file that specifies the file extension to content-type mappings, as an example.

[muratg]

Moving to post RTM for now. Ping me if you think this has to be done this week.

[Tragetaschen]

I've just tried to add a new file extension + content type to use with the file server.
There is definitely room for improvement:

var fileExtensionContentTypeProvider = new FileExtensionContentTypeProvider();
fileExtensionContentTypeProvider.Mappings[".vtt"] = "text/vtt";
var options = new FileServerOptions();
options.StaticFileOptions.ContentTypeProvider = fileExtensionContentTypeProvider;

app.UseFileServer(options);

[Tratcher]

Policy considerations:

• Include file types included in other server's default sets
• Do not include file types likely to contain server-side code or configuration (e.g. cs, cshtml, json, etc..)
• Prefer mime types with well documented standards (e.g. RFCs)

This policy should be placed in the wiki and/or in a doc comment on FileExtensionContentTypeProvider