Hi @René van Britsem ,
When I tried with the provided Forge Component Documentation I achieved the Solution.
Please check the Attached OML and Demo Link.
DEMO Link
1. Make Sure the component is available in your application

2. Use the Multiple File Upload available web block

3. Define actions for handling the web block triggers.

I'm Attaching the same here In Below.
I hope this helps.
Still you are facing the issue, Please share the OML. so that we can help you.
- Palle Vijay Bhaskar Reddy.