This use case is something that would be easy to implement as an Aikau page in Share... you've said you want to create a WebScript though, rather than a page - is there a particular reason for this? How are you hoping for your end users to access this new feature?
Generating previews is really simple with Aikau as all you need is the "nodeRef" of the content you want to preview. There is an AlfDocumentPreview widget you can use which is demostrated on the Aikau Sandpit.
There is also some useful information on the Aikau tutorial on GitHub in how you can use this widget... in fact, working through the tutorial will give you a good idea of how to create pages for Share using Aikau.
You can use the doclib.lib.js import file to help you build as much or as little of a Document Library to viewing and uploading the content.
I'd recommend that you work through the tutorial (or at least read through it) and then have a go and using the DocLib import - if you have any questions then please add follow up questions here or start a new thread.
