code: def from_path_by_ext(dir_path, ext):
          
          if isinstance(ext, (list, set, dict)): # collection of extension
              def filter(winfile):
                  if winfile.ext in ext:
                      return True
                  else:
                      return False
          else: # str
              def filter(winfile):
                  if winfile.ext == ext:
                      return True
                  else:
                      return False
  
          return FileCollection.from_path_by_criterion(
              dir_path, filter, keepboth=False)
docs: Create a new FileCollection, and select all files that extension 
          matching ``ext``::
          
              dir_path = "your/path"
              
              fc = FileCollection.from_path_by_ext(dir_path, ext=[".jpg", ".png"])