Can you explain this: https://docs.strapi.io/dev-docs/custom-fields#registering-a-custom-field-in-the-admin-panel:~:text=Though%20the%20recommended%20way%20to%20add%20a%20custom%20field%20is%20through%20creating%20a%20plugin%2C%20app%2Dspecific%20custom%20fields%20can%20also%20be%20registered%20within%20the%20global%20register%20function%20found%20in%20src/index.js%20and%20src/admin/app/js%20files.

I'm struggling to find anything that could describe how can we implement a custom field with the file without having to create a plugin, but I don't find anything on the web.