Some customers have asked if they can use their own file storage system with the app. Technically we cannot provide this functionality as a deep integration, however, there are some ways to make this work in a limited fashion.
For example, suppose you have a collection of MP3 files stored somewhere that's accessible on the internet and you want to add an MP3 player to Shopify that plays those files, but you don't want to migrate the files into Custom Fields. Consider using a URL field in Custom Fields and pasting in the URLs to your files in the Custom Fields app. From there, you could write some custom template code to use that URL in your MP3 player.
Using methods like the above do have drawbacks in that you cannot use the full potential of the app. For example, if you have images stored outside of the app, you wouldn't be able to create an image field in the app and use our image editor. You'd need to make those changes elsewhere.