Security Fixes

Multipart field names sanitization

Hardened the internal storage used by the multipart parser to collect form fields, preventing potential misuse of specially crafted field names.

File type detection memory improvement

Fixed an issue where the internal buffer used for file type detection could grow indefinitely when processing files whose content didn't match any known magic number signature. The parser now falls back to filename-based detection after a reasonable threshold, ensuring predictable memory usage during file uploads.