Support for an file upload component (image upload, PDF upload etc)

It would be handy for some use cases to have a component that represents a file upload slot. Some thoughts:
- The contract could specify how the file is sent via A2A, which already has a FilePart etc
- The client would ideally show the uploaded file name and progress in the UI
- The UI would be flexible and look different on different devices e.g. a drop zone on web, or a button to open a picker on mobile
- There should be type filtering, to allow for specifically an image picker etc. Perhaps this is a usageHint.
- On mobile, it must be possible to make this an image picker which opens the phone gallery/camera

Look at the existing component structure in the repository to understand how UI components are built and how they integrate with A2A (likely a communication layer). Examine the FilePart handling in A2A. The work involves creating a new component that adapts to web and mobile, handling file selection, upload progress, and type filtering. Start by finding similar interactive components to see their patterns. 'Done' means a component that works across platforms with the described features.

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