The content.nonAttach.mimetypes property specifies the MIME types (by default: application/pdf, image/jpeg, image/gif, image/png, image/tiff ,image/bmp) that can be viewed in a browser by clicking the Alfresco Share button View in Browser, all other file types are forced to be downloaded.
This property can be overridden, but it’s discouraged since it might cause a security breach.