Hi, is there a way to access temporary document uploaded from a start form that hasn't been submitted yet?
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In general, no. But what's your use case?
Yes, you'll need to design this in such a way that it's done on an interface that can be submitted prior to the contents of the document being processed, if that's your requirement. I try to avoid start forms in such cases (and honestly, most other cases too) as they don't add much usefulness and add a certain overhead of confusion around configuration.
My suggestion would be to have a Start Process Link from your dashboard where the user lands on a User Input Task within the process flow, uploads the document then clicks a submit button or link to "confirm" or "validate" the upload, whereupon the process will loop around and immediately back to the same interface (or a subsequent one) where they can see/use the results of whatever post-upload processing you need to do to the uploaded file. When they complete the process, they will be returned back to the dashboard with the original Start Process Link, unless they navigate away manually before that.