set_param(param, value, header='Content-Type', requote=True, charset=None, language='', replace=False)
Set a parameter in the Content-Type header. If the parameter already exists in the header, its value will be replaced with value. If the Content-Type header as not yet been defined for this message, it will be set to text/plain and the new parameter value will be appended as per RFC 2045.
Optional header specifies an alternative header to Content-Type, and all parameters will be quoted as n