I do not think Meteor has support for downloading; perhaps something is planned in the future. At the same time, I would either (assuming you can read the file using FileReader or something else):
a) paste the file in mongo and then delete it when it is saved to S3
b) save it to S3 from the client (not sure if there is a “safe” way to do this .. that is, without giving out your credentials, but it might be worth exploring).
c) [if you cannot use FileReader] use an iframe or something in POST to an external server that you can control, which handles downloads, and then communicates with it from the meteorite server.
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