Some Microsoft Word docx files uploaded to AEM Assets have thumbnails that are blurry and illegible.
This is a known product limitation.
AEM Assets does not generate docx thumbnails, instead it extracts them directly from the docx file archive. If Microsoft Word happens to generate a faulty or blurry thumbnail, the same is carried over to AEM.
As a workaround, a command line tool could be employed to generate thumbnails. For example:
The tool could be applied via the Command Line workflow process step and the configuration for this would be similar to Image Magick.