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You won't find it Form Editor, but it is possible, as I describe here: HOWTO: Attach images to a Forms 2.0 form post
As noted, it's best suited for avatar images, small text files, or perhaps a filled-in Word or PDF form that you've already provided via download (thus with a predictable size). It isn't for general-purpose file uploading. For that, you'd use a third-party service (you'd need to reconcile the outside service with Marketo unique identifiers, which is possible but you'd need to have a skilled dev on hand).
I god that.
Thank you so much.