The only way to find out how it was installed is to look at server logs from the time it was installed. It seems very unlikely that any of those logs are still available for you to look at.
Also, just a note, none of the modifications offered here are "officially approved". We don't go through any approval process with the modifications. If we hear of any rogue files like the one you found being supplied with the modification, we would remove the modification. I don't recall of ever hearing of any file like the one you found being supplied with any modification here.
Are you on your own server or on a shared server?
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