Upload an image to a note. If placeholder_id is provided and matches a {{IMG:placeholder_id}} marker in the note content, the marker is replaced with an <img> tag. If the placeholder was already uploaded (re-upload), the old image is replaced. Without placeholder_id, the image is uploaded as a plain attachment.

Supported formats: JPEG, PNG, GIF, WebP. Maximum file size: 25MB. Maximum 20 images per note. File content is validated against magic bytes to prevent spoofed MIME types.