The help sidebar for the HTTP Request node tells you how to do a file upload.
File Upload
To perform a file upload,
msg.headers["content-type"]should be set tomultipart/form-dataand themsg.payloadpassed to the node must be an object with the following structure:{ "KEY": { "value": FILE_CONTENTS, "options": { "filename": "FILENAME" } } }The values of
KEY,FILE_CONTENTSandFILENAMEshould be set to the appropriate values.