There are 3 ways that I know of
Set it manually
Reaper will read the embedded tempo (ACID)
Reaper will read it from the filename
Piano, Upright, Melody Rhythm Pop 01 Ev 4-4
80bpm A#-Gm.wav
If there is other embedded tempo in a file other that the acid byte I don't know about that
This is how it works for me: