extension_with_delimiter

Returns the extension of the filename (i.e. the substring following the last dot, excluding a dot at the beginning) with the dot that precedes it. If the filename has no extension, an empty string is returned.

ActiveStorage::Filename.new("racecar.jpg").extension_with_delimiter # => ".jpg"
ActiveStorage::Filename.new("racecar").extension_with_delimiter     # => ""
ActiveStorage::Filename.new(".gitignore").extension_with_delimiter  # => ""