curl --request POST \
  --url https://api.z.ai/api/paas/v4/files \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: multipart/form-data' \
  --form purpose=agent \
  --form file=@example-file

{
  "id": "<string>",
  "object": "<string>",
  "bytes": 123,
  "filename": "<string>",
  "purpose": "<string>",
  "created_at": 123
}