The actual content: data URL (data:mime;base64,xxx) or pre-authenticated URL
OptionaldurationDuration in seconds (for audio/video)
OptionalfileOriginal filename
OptionalheightHeight in pixels (for images/videos)
MIME type (e.g., "image/png", "video/mp4")
OptionalsizeFile size in bytes
Type category of the attachment
OptionalwidthWidth in pixels (for images/videos)
Attachment data with actual content for building ChatMessageContent. Used when loading attachments from storage to pass to AI providers.