OSM - OpenStreetMap Map File is one of the best type of files that have been used for a long time regarding XML files. You can use this type of file to actually save street map information and do much more with. There is different type of data that you also will be able to use with this file extension. You can also use this extension to design small street maps or to even scale to store streets that are worldwide. There is a web interface that can be used for showing and exporting OSM data. . This is one of the best type of extensions that can only be used for files that contain OSM data.