Attaching images and PDFs to a chat message.
Click the attachment icon in the composer, choose a file, and it appears as a preview with the file name (and image thumbnail, for images) before you send. Text is extracted automatically - from PDFs directly, and from images via OCR (optical character recognition).
Note: For a photo with no readable text (most ordinary photos), the image itself still attaches and shows in the chat - there's just no extracted text to add as context, since OCR only produces text, it doesn't describe a scene.