[5.5] Add public getter method for MIME types to be able to use them.

[mathieutu]

For now, we can't use this array of mimetypes in our apps except from filename, and it's a bit too bad.
This PR adds a simple public method to benefit of it.

[taylorotwell]

Please provide a code example of what you are referring to and what this makes possible.

[mathieutu]

If you want to see what is the MIME type of an extension:

return MimeType::get('psd') // image/vnd.adobe.photoshop

you can also need to have the extension from the MIME (I wondered to create a findExtension direct method too, but you tought me to make small PR 😅):

$mimes = MimeType::get();
return array_flip($mimes)['video/x-flv'] // flv

It's useful when you work with data-url scheme for example (you don't have the extension of the original file, in this case):

data:image/png;base64,iVBORw0KGgoAAA ANSUhEUgAAAAUAAAAFCAYAAACNbyblAAAAHElEQVQI12P4 //8/w38GIAXDIBKE0DHxgljNBAAO9TXL0Y4OHwAAAABJRU 5ErkJggg==