[MM-11777] Add WebP preview support on web (#25784)

* Add webp to webapp image extension list

* Add test webp image and test code

* Change webp test image and move path

---------

Co-authored-by: Mattermost Build <build@mattermost.com>

e2e-tests/cypress/tests/integration/channels/files_and_attachments/file_preview_image_spec.js

@@ -103,6 +103,18 @@ describe('Upload Files - Image', () => {
 
         testImage(properties);
     });
+
+    it('MM-T2264_7 - WEBP', () => {
+        const properties = {
+            filePath: 'mm_file_testing/Images/WEBP.webp',
+            fileName: 'WEBP.webp',
+            originalWidth: 640,
+            originalHeight: 426,
+            mimeType: 'image/webp',
+        };
+
+        testImage(properties);
+    });
 });
 
 function testImage(properties) {