SharePoint does not recognize .DWG AutoCAD files in document libraries, so will display a default “blank file” icon for those files. If you are comfortable with hacking it into the Docicon.xml file in the 14 hive, an on-premise farm enables you to do that. SharePoint Online, not so much.

Instead you can add the below CSS-snippet to a custom style sheet on you site, or you can paste in into a Script Editor Web Part on a specific page, typically by editing a list view page. Obviously it performs better than JavaScript hacks, but it also plays nicely with the Minimal Download Strategy (MDS) feature.