IMG-021: No File Extension Validation

[murdore]

Summary

File extension detection trusts any extension without validation.

Root Cause

Lines 448-451 trust any file extension without whitelist.

Fix

Define whitelist of valid image extensions, validate against whitelist.

Acceptance Criteria

• [ ] Define whitelist of valid image extensions
• [ ] Validate extension against whitelist
• [ ] Case-insensitive extension matching
• [ ] Reject extensions with special characters