Its called the MIDI Tuning Standard. There are 2 different forms of
it. The older one doesn't properly support tuning bank #s. They
created a second revision of it to fix that oversight and provide
some additional things (such as octave tunings). There are some
tools, such as Scala, which can be used to generate tunings. Its a
pretty complex program. I was able to export a MIDI file containing
a MIDI Tuning Standard (MTS) dump, which I successfully loaded with
FluidSynth.
http://www.huygens-fokker.org/scala/