The whole pattern hinges on clipboardData.items rather than clipboardData.files, because Safari does not populate files on paste events even when an image is on the clipboard. Walking items and filtering on kind === 'file' plus type.startsWith('image/') works in every desktop browser I have tested. The preventDefault call is what stops the textarea from inserting image.png as plain text, which is the most common bug report: "I pasted a screenshot and nothing happened, but the filename appeared." Returning the cleanup function lets React or any framework remove the listener on unmount.

Magic-byte sniffing is the only reliable way to tell HEIC apart from PNG, because Safari occasionally lies about the MIME type when a screenshot has been edited in Photos and re-saved. The ftypXXXX tag at offset 4 covers HEIC (heic/heix) and HEIF (mif1/msf1), which together cover everything an iOS user can paste. I keep the conversion path optional: most projects can show a friendly "HEIC is not supported, please paste a JPEG" rather than ship a megabyte of heic2any. Sniffing 12 bytes is fast enough that I run it on every paste without measuring.