diff --git a/src/vs/base/browser/ui/resourceviewer/resourceViewer.ts b/src/vs/base/browser/ui/resourceviewer/resourceViewer.ts
index bfc83c0b89bf4..dd4454650a06f 100644
--- a/src/vs/base/browser/ui/resourceviewer/resourceViewer.ts
+++ b/src/vs/base/browser/ui/resourceviewer/resourceViewer.ts
@@ -79,7 +79,7 @@ export class ResourceViewer {
 		let mime: string;
 		const ext = paths.extname(resource.toString());
 		if (ext) {
-			mime = mapExtToMediaMimes[ext];
+			mime = mapExtToMediaMimes[ext.toLowerCase()];
 		}
 
 		if (!mime) {