There’s a little ‘Upload’ button in the toolbar of the editor box.
You can also paste directly into the editor box if you’ve got something on your clipboard.
A sync chain is like an ‘account’, more or less, you can attach devices to that chain and sync data into and out of it. Understand your concerns about syncing data externally, Brave’s Sync is pretty good in that your data upload locally first before uploading – so neither they nor anyone else have access to the actual content.
The reason I asked about which version of Windows: Windows 7 is so far out of support, that many SSL certificates used widely on the Internet expired recently and Win7 wouldn’t automatically get updated certificates. We had a ton of posts here of all sorts of random problems, some of them quite obscure, that could be traced back to that issue.
See here for a workaround: For users encountering NET::ERR_CERT_DATE_INVALID Error
It may not be your issue under the hood, but it’s an easy target.