When you upload a file from online storage to a website, does the file get stored in the Mac? Online Storage can be dropbox, google drive whatever. You only have files saved there but not on the mac. But when you upload from the online storage directly to a website, i am guessing the file has to go thru the mac, and does it get stored somewhere in the mac, then?
If you get prompted to download a file then it would be saved on your mac which I doubt that this is the case since these websites usually build these based on API's available from Dropbox or Google Drive etc. where you'd be prompted to logon and usually even with some sort of authorization that the website wants gain access to your personal information