[#67992430] fixed merged manifest sending as an attachment

From the DEV guide of the spreadsheet gem:
"Although it is theoretically possible, it is not recommended to write the resulting Document back to the same File/IO that it was read from."
So, I had to create a new file and send that one as an attachment.
TODO: It would be better if we could create a stream and send it back to the browser, but it always sent null bytes for me, even though I rewinded it to the beginning of the stream before sending it...