Sets DMI files to open with image preview (#15945)

Currently, DMI files try to open in the text editor, despite being basically PNG files with extra header data.
This default config option would mean it opens them as an image preview within VSC.

.vscode/settings.json

@@ -0,0 +1,8 @@
+{
+	"workbench.editorAssociations": [
+		{
+			"filenamePattern": "*.dmi",
+			"viewType": "imagePreview.previewEditor"
+		}
+	]
+}