Review `files` implementation

[rodber]

Files are being hinted by Controller using ArrayTypeParameterInterface:

public static function acceptFiles(): ArrayTypeParameterInterface

This is limiting the design and not providing a clear type hint as it supports any ArrayType and it should work only with FileInterface parameters. The syntax should allow to define multiple files easily:

public static function acceptFiles()
{
    return files(
        photo: file(mime: 'image/jpeg'),
        signature: file(mime: 'application/pdf'),
    );
}

This requires a new array type for files, perhaps support for optional and required parameters. Similar to the other array shapes on Parameter.