I'm pretty sure this could be accomplished via CSS/JS, however I'd thought I'd check here first before jumping on the script train.
I think this will actually help you out a lot: http://www.raymondcamden.com/index.cfm/2013/9/10/Adding-a-file-display-list-to-a-multifile-upload-HTML-control
I hope this answers the question precisely for you. I think I may use this for Forms I create in the future as it would be convenient
We actually support this out of the box as of 10.3, see this KB: https://support.laserfiche.com/kb/1013903/list-of-changes-for-laserfiche-forms-10-3
What is the use case for this?
Are you trying to display it within the same form that it is uploaded from? Or are you trying to view uploads from a previous submission inside the same business process?
Same form. Customer requested the functionality of an image preview prior to uploading the document.
Well...Scripting to add in a document to the file upload has been annoying from my experience. I have. If you are headed down that path, I recommend looking into the accessibility of information from the HTML Input tag when "type=file" as that is how the file-upload forms activity is created on the page.
Might want to try using an iFrame window to display the document inside the form after you have the other bits figured out.