Add File Type and Size Validation

[lephorx]

Title: Implement file upload validation (type, size, security)

Description:
Add comprehensive validation for uploaded files to prevent security issues and resource abuse.

Tasks:

• Validate file extensions against allowlist
• Implement file size limits (configurable)
• Add MIME type validation
• Scan for malicious file signatures
• Validate file headers match extensions
• Add virus scanning (optional)
• Return descriptive error messages