That isn’t exactly a mixer file, but you are right that it is an airframe RC file that overrides part of the default airframe configuration. The file you posted loads a mixer on line 3 (set MIXER FMU_quad_x). If this referenced mixer doesn’t exist, it would cause a problem, which was probably what was happening to you. Another possibility is that it was loading a file called FMU_quad_x.main.mix on the SD card under /etc/mixers, but not sure.
https://dev.px4.io/master/en/airframes/adding_a_new_frame.html