Add a comprehensive MIME registry

[straight-shoota]

@asterite on #2470 (comment):

We should probably have some Mime class/module as a registry for this, similar to Go's (since we provide an HTTP::Server in the standard library we will probably want this in the standard library too).

There are three shards who provide a MIME registry:

• spalger/crystal-mime
• jwaldrip/mime-types.cr
• Rinkana/Crystal-MimeTypes

And ysbaddaden/frost has a extensive MIME registry.

They all rely on a custom mime type database in form of a JSON file included in the shard. While this can be required in some cases, I feel like the stdlib implementation should probably just rely on the mime types registry of the operating system (like the Go example).

[asterite]

We can start with a Mime registry with some default extensions and a way to add more extensions to the registry. Then in a separate PR we could load mime types from the OS if available.

[asterite]

And the existing shards would just add to the standard registry instead of providing alternative registries.

[ysbaddaden]

Note that on linux (at least debian/ubuntu) a mime type database can usually be found at /etc/mime.types as a list of:

# comment
mime/type    ext ext2 ext3

[RX14]

Perhaps an official shard to add magic-guessing to the registry too wouldn't be a terrible idea.

[asterite]

Go uses:

var typeFiles = []string{
  	"/etc/mime.types",
  	"/etc/apache2/mime.types",
  	"/etc/apache/mime.types",
}

[ysbaddaden]

Perhaps an official shard to add magic-guessing to the registry too wouldn't be a terrible idea.

Perhaps it should be in stdlib, or the whole mime registry be a mime-types shard (nothing in stdlib).

Go has a bunch of locations for each platform (windows registry, different places across the filesystem on posix systems):
https://golang.org/src/mime/

[straight-shoota]

As long as HTTP::Server is in the standard library, it also needs a MIME registry.
I agree that it should have a small set of default mime types baked in, try to load a mime database from known system locations and provide a means to add/change mime type entries programatically. Maybe even allow to load a mime type database from a customizable path, similar to the customizable timezone database location using ENV["ZONEINFO"] (though this probably shouldn't use an environment variable).