Multi-track (type 1) MIDI files can contain tempo tracks. When you drag a type 1 MIDI file into Reaper, it should prompt you for whether or not you want to import the tempo map as well. If you're exporting MIDI files from another app, check to see if there are options for exporting a tempo map, or exporting type 1 vs type 0 MIDI. (Exporting type 1 MIDI from Reaper will be supported in the next release.)