[Feature]: Allow document upload

### Pre-flight Checklist

- [x] I have searched existing issues and this hasn't been requested yet.

### Problem or Motivation

Currently there is no "+" button to upload documents to AG-UI. In ADK it is already supported, any way or solution to make this feature available in AG-UI?

Here is the screenshot in ADK. It works natively

### Proposed Solution

It would be great if we have it for ADK and also for other Agentic frameworks

### Alternatives Considered

_No response_

### Additional Context

_No response_