docs: def tfclasses():
      """
      A mapping of mimetypes to every class for reading data files.
      """
      # automatically find any subclasses of TraceFile in the same
      # directory as me
      classes = {}
      mydir = op.dirname(op.abspath(inspect.getfile(get_mimetype)))
      tfcls = {"<class 'aston.tracefile.TraceFile'>",
               "<class 'aston.tracefile.ScanListFile'>"}
      for filename in glob(op.join(mydir, '*.py')):
          name = op.splitext(op.basename(filename))[0]
          module = import_module('aston.tracefile.' + name)
          for clsname in dir(module):
              cls = getattr(module, clsname)
              if hasattr(cls, '__base__'):
                  if str(cls.__base__) in tfcls:
                      classes[cls.mime] = cls
      return classes