When you open Documents, a preview thumbnail is displayed for documents that are stored locally, on Google Drive or ownCloud. Those stored in OneDrive show as missing (or blank) preview thumbnails.
If you download a OneDrive document to local storage through the web interface, a thumbnail will be generated.
The local copy of a document downloaded from an online account will lose its ability to be updated online. If you want to continue to edit it online, it is better not to download it.
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