This example explains how to get a value with jQuery from a select and set another input value dynamically ref. https://wordpress.org/support/topic/most-elegant-way-to-achieve-this/
you can enable the send withdrawal of the form (like in gmail) and abort the submission. Add this code to functions.php ref. https://wordpress.org/support/topic/withdrawal-button/
This is an example created some times ago for a support question into contact form 7 support forum, but could be very useful in order to show the uploaded images into a form with a input type file. ref. https://wordpress.org/support/topic/image-upload-preview-before-submit/
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