docs: document fileExtension

README.md

@@ -44,6 +44,7 @@ The matcher takes two optional arguments:
 - `filepath`: path to the file whose content should be matched, e.g. `expect(content).toMatchFile(path.join(__dirname, 'output.md'))`
 - `options`: additional options object for the matcher, with following properties:
   - `diff`: options for [`jest-diff`](https://github.com/facebook/jest/tree/master/packages/jest-diff)
+  - `fileExtension`: optional file extension to use for the snapshot file.
 
 You should also [exclude the output files from Jest's watcher](https://jestjs.io/docs/en/configuration#watchpathignorepatterns-arraystring) so that updating the snapshot doesn't re-run the tests again.
 

__tests__/__file_snapshots__/matches-content-of-file-on-disk-with-file-extension-0.md

@@ -0,0 +1 @@
+# this is a another test

__tests__/index.test.js

@@ -14,6 +14,12 @@ it('matches content of file on disk without filename', () => {
   expect(`# this is a another test`).toMatchFile();
 });
 
+it('matches content of file on disk with file extension', () => {
+  expect(`# this is a another test`).toMatchFile(undefined, {
+    fileExtension: '.md',
+  });
+});
+
 it('matches binary content of file on disk', () => {
   expect(
     fs.readFileSync(path.join(__dirname, 'minimal.pdf'), 'binary')