The delegate you mention would be used if you need to add support for new types of files, but if you only need to upload a file it shouldn't be necessary to add a new handler. In your case, you should be able to upload files using existing services.
In particular, I'd suggest that you look into src/core/features/fileuploader. From plugins, you have access to most services. You can see which ones are available looking at src/core/features/compile/services/compile.ts. For example, it is importing
CORE_FILEUPLOADER_SERVICES which exposes
If you want to see how these services are used in the app, I'd suggest that you clone the source code of the app and search for usages. If you want to understand the codebase of the app better, I'd suggest that you read the Moodle App Development Guide. You don't need to read this to develop plugins, but if you're working on a use-case that is not explicitly documented in the plugins guide, you'll probably have to dig into the source code and it will be useful to understand it.
Also, check out this previous post in the forum about a similar topic: Taking a Picture / File Upload from Moodle App in custom plugin.