Failed to download Firmware - AP 3.7.1 on 1.3.49

#### Issue details
Able to load firmware 3.8.0 onto Pixhawk 2.1, but cannot load ArduPlane 3.7.1. When I try, after plugging the Pixhawk 2.1 back in after the first step of the process, it fails with a "Failed to Download" error.

#### Version
1.3.49 downloading firmware AP 3.7.1

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

#### Airframe type
_What type of airframe (flying wing, glider, hex, Y6, octa etc)_

#### Hardware type
_What autopilot hardware was used? (pixhawk, pixracer, PX4FMU etc)_

Pixhawk 2.1

#### Logs
_Please provide a link to any relevant logs that show the issue_

Reproduce the firmware download flow in Mission Planner 1.3.49 with a Pixhawk 2.1, comparing ArduPlane 3.7.1 with the working 3.8.0 upload. Check the available download or application logs, then identify why the first-step reconnect fails; done means ArduPlane 3.7.1 downloads successfully or the failure is clearly diagnosed.

Written by the indexing model from the issue text.