Revert "fix loop indentation (f*ck tabs!)"

This reverts commit 3ca69f25ababe1743b1bc5fb59f2466c57a6eb1a.
The code works as intended, but the author was very clever with
Python's scoping ("ext" is not actually scoped to the loop, it
continues to be defined thereby ending up being "the approved
extension of the first file in the list). I shall fix this
to be self-explanatory.