Looks like XX in the "Button-XX" entries are only meant to be continuous indexes, I changed the file to this:
Button-73=280,(Y)
Button-74=281,(X)
Button-75=279,(
Button-76=278,(A)
Button-77=9,Tab
Button-78=303,(L)
Button-79=305,(R)
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And now (A) works to play a file in...