The problem is visible in rdar://100336472 , where the results page shows "ref mismatch" but the URL contains an extra dot: `[...]attachment-wrapping-action-expected-mismatch..html`, making it impossible to view the image.
The code in results.html's imageResultLinks looks like:
>            let testExtension = Utils.splitExtension(testResult.name)[1];
...
>                result += TestResultsController.resultLink(this._resultsController.layoutTestsBasePath() + testPrefix, '-expected-mismatch.' + testExtension, 'ref mismatch');
...
>                    result += TestResultsController.resultLink(this._resultsController.layoutTestsBasePath() + testPrefix, '-expected.' + testExtension, 'reference');

The `testExtension` variable contains ".html", so we don't need to add one before.

<rdar://106785429>