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awesome-selfhosted
A list of Free Software network services and web applications which can be hosted on your own servers
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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
But to answer your question: me and a friend of mine are currently working on a self-hosted file management app. We're building it as a NextCloud alternative, but it is essentially a file explorer with a JSON API,so it sounds like it could fit your use case. Do note that it is currently still in beta, but we're slowly nearing version 1.0 (and I don't expect the existing API to change much). If you're interested: https://github.com/keydrive/keydrive
I'm trying to find a self hosted solution that is a file explorer, but at the same time that can be programmatically queried and returns JSON list of files. I have my lab all running in dockers (Synology NAS and Raspberry Pi). I've been testing some options, found a very comprehensive list on Github here, however none of the options has an API (none that I could find anyways).