code: def get_extension(filepath, check_if_exists=False, allowed_exts=ALLOWED_EXTS):
      
      if check_if_exists:
          if not op.exists(filepath):
              raise IOError('File not found: ' + filepath)
  
      rest, ext = op.splitext(filepath)
      if ext in allowed_exts:
          alloweds = allowed_exts[ext]
          _, ext2 = op.splitext(rest)
          if ext2 in alloweds:
              ext = ext2 + ext
  
      return ext
docs: Return the extension of fpath.
  
      Parameters
      ----------
      fpath: string
      File name or path
  
      check_if_exists: bool
  
      allowed_exts: dict
      Dictionary of strings, where the key if the last part of a complex ('.' separated) extension
      and the value is the previous part.
      For example: for the '.nii.gz' extension I would have a dict as {'.gz': ['.nii',]}
  
      Returns
      -------
      str
      The extension of the file name or path