Returns a temporary download URL for one file on an envelope: the original PDF or Word file, an attachment, or the signed (sealed) PDF. Set objectType to envelopes, objectId to the envelope ID and fileId to a fileId from the envelope (content[].originalPdfFileData.fileId, content[].originalWordFileData.fileId, content[].sealedFileData.fileId or attachmentContent[].fileId). The signed file exists once the envelope is SEALED (the envelope.sealed webhook announces this); each document has its own signed file. The URL is valid for 600 seconds and needs no API key; request a new one when it expires.