[fxcop] preview handler common (#6762)

* FxCop adjustments

* initing due to change for abstract

src/modules/previewpane/SvgThumbnailProvider/SvgThumbnailProvider.cs

@@ -225,7 +225,7 @@ namespace Microsoft.PowerToys.ThumbnailHandler.Svg
             }
 
             string svgData = null;
-            using (var stream = new StreamWrapper(this.Stream as IStream))
+            using (var stream = new ReadonlyStream(this.Stream as IStream))
             {
                 using (var reader = new StreamReader(stream))
                 {