[galaxy-dev] Adding New filetype

Hi Galaxy-Dev, I have a a special set of .txt files which contains some comments and tabular feilds. I want to make them as a different group so that I can list them without any other .txt files. I went through the Add New Data Type section.But I didnt get a clear idea of how to do it? The datatypes_conf.xml file already has an entry for txt. What is mime type and how we know to which class I have to add the new file type? Deepthi