code: def guess_mimetype(blob):
      
      mimetype = 'application/octet-stream'
      # this is a bit of a hack to support different versions of python magic.
      # Hopefully at some point this will no longer be necessary
      #
      # the version with open() is the bindings shipped with the file source from
      # http://darwinsys.com/file/ - this is what is used by the python-magic
      # package on Debian/Ubuntu. However, it is not available on pypi/via pip.
      #
      # the version with from_buffer() is available at
      # https://github.com/ahupp/python-magic and directly installable via pip.
      #
      # for more detail see https://github.com/pazz/alot/pull/588
      if hasattr(magic, 'open'):
          m = magic.open(magic.MAGIC_MIME_TYPE)
          m.load()
          magictype = m.buffer(blob)
      elif hasattr(magic, 'from_buffer'):
          # cf. issue #841
          magictype = magic.from_buffer(blob, mime=True) or magictype
      else:
          raise Exception('Unknown magic API')
  
      # libmagic does not always return proper mimetype strings, cf. issue #459
      if re.match(r'\w+\/\w+', magictype):
          mimetype = magictype
      return mimetype
docs: uses file magic to determine the mime-type of the given data blob.
  
      :param blob: file content as read by file.read()
      :type blob: data
      :returns: mime-type, falls back to 'application/octet-stream'
      :rtype: str