How can `firebase.functions().httpsCallable` submit files to `functions.https.onCall`?

Question

Background:

1. We want to allow users to upload files onto our website
2. The files need to be validated and modified before they are writt

Answer 1

That's not really an intended use case for callable functions. Callables are meant to send a JSON payload to the server, not a file. If you really want to send a (small) file to a function, you'll need to base64 encode it as a string, and put that string in the JSON payload, then decode it in the function.

Instead, consider allowing the client to upload directly to storage, then use a function to validate it with a storage trigger, and delete it if it's not valid.