Maybe I'm mistaken about "background downloads" being a potential problem though. After all, extensions were able to do something similar in the past (i.e., download a file without a "Save As" dialog or unknown content type dialog opening). It just seems like someone more knowledgeable about webextensions might object to exposing this directly as a parameter.
Bug 1759231 Comment 6 Edit History
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Maybe I'm mistaken about "background downloads" being a potential problem though. After all, extensions were able to do something similar in the past (i.e., download a file without a "Save As" dialog or unknown content type dialog opening). It just seems like someone more knowledgeable about webextensions might object to exposing this directly as a parameter. Also, it seems like whether there needs to be an affordance for downloads created by extensions, and whether there could be a delay on that, might be questions for the UX team.