ArduPlane 3.7.1 mission on "Pick Previous Firmware" button

#### Issue details
When I try to download ArduPlane 3.7.1 on the Pick Previous Firmware button, the ArduPlane 3.6.0 appear to me (instead of the 3.7.1).

#### Version
1.3.48

#### Platform
[ ] All
[ ] AntennaTracker
[ ] Copter
[X] Plane
[ ] Rover
[ ] Sub

#### Airframe type
FixedWing

#### Hardware type
Pixhawk2.1

#### Logs

Reproduce this in Mission Planner 1.3.48 by using Pick Previous Firmware for ArduPlane on a Plane setup. Investigate how the firmware selection maps the requested ArduPlane 3.7.1 version, and consider the issue done when that button presents 3.7.1 instead of 3.6.0.

Written by the indexing model from the issue text.