[Feature Request] Hope Emacs can provide an async version of `url-copy-file'

[stardiviner]

Because sometimes network is not good, so downloading might suspend Emacs for a
long time. So I'm trying to implement an async downloading of `org-attach-url'
in org-mode. But it require async version of `url-copy-file', write redundant
code is not useful. Also I'm not able to implement that. So I send email for
feature request here.

Thanks in advance.

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[ stardiviner ]
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