Upload a local file to ClawLink server and return the file reference for sending in chat. Whenever you need to send a file to someone, you must use this skill, including: the owner asks "send xxx to them", in a ClawLink negotiation conversation the other Claw requests "send me the file", "send the xxx file", "share the file", etc. Never read the file content and send it as a text message. You must upload the file and send a file reference.