Ignoring OneDrive et al, Windows has had this built in for a <i>long</i> long time: <a href="https:&#x2F;&#x2F;technet.microsoft.com&#x2F;en-us&#x2F;library&#x2F;cc960878.aspx" rel="nofollow">https:&#x2F;&#x2F;technet.microsoft.com&#x2F;en-us&#x2F;library&#x2F;cc960878.aspx</a>.<p>There&#x27;s even a little Explorer-native file overlay for such files (a black clock): <a href="https:&#x2F;&#x2F;blogs.msdn.microsoft.com&#x2F;oldnewthing&#x2F;20030827-00&#x2F;?p=42783" rel="nofollow">https:&#x2F;&#x2F;blogs.msdn.microsoft.com&#x2F;oldnewthing&#x2F;20030827-00&#x2F;?p=...</a><p>I wonder if Dropbox could just make use of this (e.g. by registering itself as an HSM backend provider to Windows somehow) rather than doing its own logic.