In the bottom half of the file, under [ExportFilters], cut and paste the format you want to be the default to the top of the list. Rearrange as needed, being sure not to remove or duplicate any information.
[ExportFilters] dxf = "DXF AutoCAD DXF Drawing", ioDxf, ExportDXF, 3S, *.dxf adf = "ADF Arc/Info Binary Grid", ioAdf, ExportADF, U, *.adf am = "AM Amira Mesh", ioAmira, ExportAMIRA, URC, *.am *.col ....
Save and close the file and restart Surfer.
Updated September 11, 2017