Are they just JS and Image files? If so, you have a few options:
You can add them as Resources to your application (resources do not necessarily need to be images - they can be anything from images, fonts, JS, etc.) and then load them via the {resource} plugin.
Any reason you're not using the hosted version? Leaflet seems to offer several free CDN's for hosting: https://leafletjs.com/download.html - using a hosted option means you only need to keep track of the version to include in your app, rather than needing to update the entire file itself. It'll also reduce the size (though, in this case, not likely enough to make a big difference).