Website opens tar.gz download?????

Okay, this is friggin' weird. I am helping someone with their website, but today, when I type in the URL, on TWO different computers -- on any browser, I get a dialog box open asking me to save download.tar.gz (see attached file). What's with that? I then tried using 4G on my phone and it opened fine and it opens fine with wifi (same router as computers) on my phone, too. Can anyone please tell me what is going on? Does it have something to do with default file types? If so, why wouldn't this happen on other websites? It's only this particular website. Thaks in advance.
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