[Feat]: Does not support file uploads

### Is your feature request related to a problem? Please describe.

Many A2A implementations (and the spec) support/require file uploads from the A2A client to A2A server. Currently the a2a-inspector does not support any way to upload a file.

### Describe the solution you'd like

Provide a file-upload mechanism in addition to text chat.

### Describe alternatives you've considered

_No response_

### Additional context

_No response_

### Code of Conduct

- [x] I agree to follow this project's Code of Conduct

Look at the existing chat interface in the frontend to understand how messages are sent. Investigate the API endpoints that handle client-server communication. Determine where file upload logic should be added, likely involving form handling and multipart data. Check if there are existing tests for the chat feature to see how to test file uploads.

Scritto dal modello di indicizzazione a partire dal testo della issue.