[Component amendment]: File upload

## Component/pattern to amend

File upload

## Visual


Live example: [MAAS File Upload component](https://canonical.github.io/maas-react-components/?path=/docs/components-fileupload--docs)

## Context
I’m proposing the inclusion of a file upload component in Vanilla. This component already exists and is actively used in MAAS, and it’s increasingly being adopted across other Canonical apps and sites.
The need for a consistent file upload experience is becoming more common across products (e.g. when uploading configuration files, metadata, scripts, or assets).
Upstreaming this component into Vanilla would promote consistency, improve usability, and reduce the need for teams to build bespoke or inconsistent solutions.