Sorry, that's not customizable without modifying the source. We could add such an icon in a future version, though I'm not sure how common it is to store Word files in Zotero.
In my experience working with folks using Zotero for systematic reviews, it is very common to store Word documents, Excel and CSV files, text files (with various extensions), and data files in various formats. Word and Excel/CSV are especially common as online supplemental materials. It would be awesome if Zotero could provide icons or even use the system icons.
It is very common to use Zotero to save Word files and even photos and Excel along with articles, and this is one of the great features of Zotero. It would be great if Zotero would display different icons for the mentioned files.
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Support for Office (Word,Excel, possibly Powerpoint) would be beneficial.