A MID file in GIS files category is a GIS data file generated with MapInfo spatial data analysis and visualization software. It is usually created alongside the corresponding MapInfo Interchange Format (MIF) file and has the same name. MID files store large data sets that contain attribute information for the objects in the map and is imported in the object’s attributes.
MID File Format - More Information
MID files are saved to disc in binary file format. Attributes in the MID files is saved in the same order as that of the corresponding MIF file. For example, the third geometric entity in the .mif file has the attributes stored in the third row of the .mid file.