Use blobs to set the svg source of an image in the image viewer.

This avoids errors that apparently are typical with using the built in btoa() with unicode text.

Fixes #10014

packages/imageviewer/src/widget.ts

@@ -178,11 +178,15 @@ export class ImageViewer extends Widget implements Printing.IPrintable {
     if (!cm) {
       return;
     }
+    const oldurl = this._img.src || '';
     let content = context.model.toString();
-    if (cm.format !== 'base64') {
-      content = btoa(content);
+    if (cm.format === 'base64') {
+      this._img.src = `data:${this._mimeType};base64,${content}`;
+    } else {
+      const a = new Blob([content], {type: this._mimeType});
+      this._img.src = URL.createObjectURL(a);
     }
-    this._img.src = `data:${this._mimeType};base64,${content}`;
+    URL.revokeObjectURL(oldurl);
   }
 
   /**