Get a URL to upload a creative's image, video or audio file to. The file is kept with the campaign's project.

Needs the campaigns:write scope, like every campaign write.

Upload with the method the answer names, before expires_at (15 minutes):

• POST: a multipart form to upload_url carrying every entry of fields, then the file in a field named file. Used for files of 5 MB or less.
• PUT: the file's bytes to upload_url, with Content-Type set to the content_type you sent. Used for larger files.

Then call Confirm creative upload with the s3_key.