Seth's comment is correct, it uses the OS default.
But a more complete answer is you can also customize the default behavior for non-default editors through Tools > Options > Custom Actions. In theory at least, because with v220.127.116.11 it doesn't work too well because my editor remains in the background. But that's better than nothing, if you can't change the system default (can be inconvenient for HTML, for example). In my experience you have to restart SourceTree for this to take effect, by the way (the program doesn't inform you of that, which made me waste time on pointless "troubleshooting").
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