throw when image extension not provided

README.md

@@ -279,11 +279,9 @@ The JS snippet must return an _image object_ or a Promise of an _image object_,
 
 * `width` in cm
 * `height` in cm
-* `thumbnail` _[optional]_: path to png to use when svg is not supported or shown in preview (svg only) 
-* `data` _[optional]_: either an ArrayBuffer or a base64 string with the image data
-* `extension` _[optional]_: e.g. `.png`
-
-Either specify the `path` or `data` + `extension`.
+* `thumbnail` _[optional]_: non-SVG image to use when SVG is not supported or shown in preview (only relevant when image is SVG). See example below.
+* `data`: either an ArrayBuffer or a base64 string with the image data
+* `extension`: e.g. `.png`
 
 Images can also be SVG. These needs a thumbnail (fallback) to be specified along with the image data.
 

src/__tests__/images.test.ts

@@ -61,4 +61,43 @@ import { Image } from '../types';
       expect(result).toMatchSnapshot();
     });
   });
+
+  describe(sbStatus, () => {
+    it('002: throws when thumbnail is incorrectly provided when inserting an SVG', async () => {
+      const template = await fs.promises.readFile(
+        path.join(__dirname, 'fixtures', 'imagesSVG.docx')
+      );
+      const thumbnail = {
+        data: await fs.promises.readFile(
+          path.join(__dirname, 'fixtures', 'sample.png')
+        ),
+        // extension: '.png', extension is not given
+      };
+
+      const opts = {
+        noSandbox,
+        template,
+        data: {},
+        additionalJsContext: {
+          svgImgStr: () => {
+            const data = Buffer.from(
+              `<svg  xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+                                  <rect x="10" y="10" height="100" width="100" style="stroke:#ff0000; fill: #0000ff"/>
+                              </svg>`,
+              'utf-8'
+            );
+            return {
+              width: 6,
+              height: 6,
+              data,
+              extension: '.svg',
+              thumbnail,
+            };
+          },
+        },
+      };
+
+      return expect(createReport(opts)).rejects.toBeInstanceOf(Error);
+    });
+  });
 });

src/processTemplate.ts

@@ -678,6 +678,11 @@ const processEndForIf = (
 };
 
 const imageToContext = (ctx: Context, img: Image) => {
+  if (!(typeof img.extension === 'string')) {
+    throw new Error(
+      'When providing an image, you need to specify the extension (e.g. `.png`).'
+    );
+  }
   ctx.imageId += 1;
   const id = String(ctx.imageId);
   const relId = `img${id}`;