code: func FilesWithExtensions(exts ...string) Func {
  	return func(path string, fi os.FileInfo) bool {
  		if fi.IsDir() {
  			return false
  		}
  		for _, ext := range exts {
  			if pathpkg.Ext(path) == ext {
  				return true
  			}
  		}
  		return false
  	}
  }
docs: // FilesWithExtensions returns a filter func that selects files (but not directories)
  // that have any of the given extensions. For example:
  //
  // 	filter.FilesWithExtensions(".go", ".html")
  //
  // Would select both .go and .html files. It would not select any directories.