[Feature] Migrate image/file upload feature from CoPaw to WebUI

[Eggiverse]

Problem Description

Currently webui does not support uploading images or files.
This feature is already in CoPaw: agentscope-ai/QwenPaw#1772

Proposed Solution

Add similar features from CoPaw to webui

Affected Component(s)

• Engine
• Sandbox
• Common
• Tools
• Documentation
• Other: webui_________

Alternatives Considered

[Describe any alternative solutions you've considered]

Additional Context

[Any other context, screenshots, or examples about the feature request]