The speaker is telling Mitch what the present looks like at some time in the future:
The Cambridge Dictionary explains:
The present simple is used to talk about the future (I work tomorrow)
Her birthday falls on a Friday next year. ( a known fact about the future)
She has her driving test next week, does she? ( a fixed arrangement)
The train arrives at 20.12. ( a timetable)
What time does their flight to Seoul leave? ( a schedule)
Your example is an example of 1. above
when we get the files,
The "when" = at the time that. The present tense imagines that the time has arrived and that time becomes the present referent point for the action.
then we've got you.
The "then" = and, immediately, as a result.
The tense here is related to the "when" clause. Again, the present tense imagines that the time has arrived and that becomes the present referent point for the action.
You've proved yourself when you hand us your files.
This is in the past because the action of proving himself has ceased as soon as "they" have the files, and having the files is in the present.