getFileExtension = fileName => {
    const dotPos = fileName.lastIndexOf(".");
    if (dotPos < 1) {
      return "";
    }
    return fileName.slice(dotPos);
  }
""                        -> ""
"noExtension"             -> ""
"file.txt"                -> ".txt"
"file.with.many.dots.doc" -> ".doc"
".gitignore"              -> ""

@param { string } fileName - the file name
@returns { string }