module CodeRay::FileType

def [] filename, read_shebang = false

That means you can get filetypes from files that don't exist.
The file itself is only accessed when +read_shebang+ is set to true.

+filename+ is a relative or absolute path to a file.

Try to determine the file type of the file. 
def [] filename, read_shebang = false
  name = File.basename filename
  ext = File.extname(name).sub(/^\./, '')  # from last dot, delete the leading dot
  ext2 = filename.to_s[/\.(.*)/, 1]  # from first dot
  
  type =
    TypeFromExt[ext] ||
    TypeFromExt[ext.downcase] ||
    (TypeFromExt[ext2] if ext2) ||
    (TypeFromExt[ext2.downcase] if ext2) ||
    TypeFromName[name] ||
    TypeFromName[name.downcase]
  type ||= shebang(filename) if read_shebang
  
  type
end

def fetch filename, default = nil, read_shebang = false

is returned.
If the filetype cannot be found, the +default+ value

This works like Hash#fetch. 
def fetch filename, default = nil, read_shebang = false
  if default && block_given?
    warn 'Block supersedes default value argument; use either.'
  end
  
  if type = self[filename, read_shebang]
    type
  else
    return yield if block_given?
    return default if default
    raise UnknownFileType, 'Could not determine type of %p.' % filename
  end
end

def shebang filename 

def shebang filename
  return unless File.exist? filename
  File.open filename, 'r' do |f|
    if first_line = f.gets
      if type = first_line[TypeFromShebang]
        type.to_sym
      end
    end
  end
end