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Sandstorm
Sandstorm is a self-hostable web productivity suite. It's implemented as a security-hardened web app package manager.
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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.
Maybe https://github.com/filebrowser/filebrowser
Is this it? https://github.com/sandstorm-io/sandstorm
Different solutions may be better for the different file types than a single download centre for everything. For example, for APKs, there's f-droid. You could selfhost fdroidserver and add your own repo to the f-droid client. This way, the APKs are distributed via an app store that's searchable and can be configured to auto update.