Add a FileUploader control for Web projects

Add a designer-visible FileUploader control to Web projects so applications can let users choose or drop files and upload them into the private server session.

The control should support single and multiple file selection, file-type hints, progress, cancellation, useful failure reporting, and safe access to completed uploads through FileSystemItem. This would enable common workflows such as image submission, CSV import, document processing, and attachments without requiring application authors to build browser transport infrastructure.