[BUG] Add MIME type validation for file uploads using magic numbers (#330)

[BhakktiGautam]

📌 Closes Issue

Closes #330

🐛 Problem

Current validation only checks file extensions. Malicious files renamed as .pdf can bypass validation.

✅ Solution

Implement magic number (MIME type sniffing) validation that reads file headers.

📁 Files Changed

File	Change
backend/utils/mime_validator.py	New
backend/utils/decorators.py	Updated
backend/requirements.txt	Updated
backend/blueprints/pdf.py	Added decorator
backend/blueprints/image.py	Added decorator
backend/blueprints/removebg.py	Added decorator
backend/blueprints/pdf_to_docx.py	Added decorator
backend/blueprints/docx_to_pdf.py	Added decorator

✅ Rule Compliance

• No data storage
• No external APIs
• Pure file manipulation

Request

• Please add GSSoC label
• Please add bug, security, level:beginner labels

Ready for review! 🚀

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[BhakktiGautam]

@Durgeshwar-AI

PRs #337, #339, #349 and #364 are ready to merge! 🚀

✅ All conflicts resolved
✅ Project rules followed
✅ GSSoC labels needed

Please review and merge when you get a chance. Thanks! 🙏