Verify Content-Type for uploads and block potentially dangerous file types

[imDarshanGK]

Ensure file uploads are checked for allowed MIME types and extensions. Reject executable files and document allowed types.

[Vikrant0207]

@imDarshanGK pls assign this issue to me

Implementation Plan:

• Add an allowlist for safe MIME types and file extensions in config.
• Install a file-type detection library (python-magic or filetype).
• Create a validate_upload() utility to:
  - detect actual file type from file signature,
  - compare MIME type + extension,
  - reject dangerous files (.exe, .sh, .bat, etc.).
• Integrate validation into all upload endpoints before saving files.
• Add filename sanitization, file size limits, and proper error handling (400 Bad Request).
• Log rejected uploads without storing file contents.
• Write unit and integration tests for valid files, spoofed MIME types, and blocked executables.
• Add documentation/config support for updating allowed file types easily.