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There's a LOT on the line when Jake Paul and Anthony Joshua enter the boxing ring in 17 days -- not only is El Gallo hoping to pull off the unthinkable and beat a former heavyweight champion ... but one very famous bettor is looking to cash in BIG on an upset.

TMZ Sports is told ... former Sony Pictures head Jon Peters is all-in on the YouTuber-turned-fighter's chances against A.J. on Dec. 19 -- to the tune of a whopping $5 MILLION wager. Getty

So ... why all the confidence in the Disney kid?? We're told Peters and Paul recently had a lengthy chat about the bout ... and the film producer (with flicks like "Flashdance," "A Star Is Born," and "Batman" to his name) was fully convinced.

We're told Paul assured Peters he's "gonna kick the s*** outta [Joshua]" at the Kase

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