Use an alternative way to detect image

When parsing content to retrieve images to save locally, we only check for the content-type of the image response.
In some case, that value is empty.
Now we’re also checking for the first few bytes of the content as an alternative to detect if it’s an image wallabag can handle.
We might get higher image supports using that alternative method.

tests/Wallabag/CoreBundle/Helper/DownloadImagesTest.php

@@ -157,4 +157,29 @@ class DownloadImagesTest extends \PHPUnit_Framework_TestCase
 
         $this->assertFalse($res, 'Absolute image can not be determined, so it will not be replaced');
     }
+
+    public function testProcessRealImage()
+    {
+        $client = new Client();
+
+        $mock = new Mock([
+            new Response(200, ['content-type' => null], Stream::factory(file_get_contents(__DIR__.'/../fixtures/image-no-content-type.jpg'))),
+        ]);
+
+        $client->getEmitter()->attach($mock);
+
+        $logHandler = new TestHandler();
+        $logger = new Logger('test', array($logHandler));
+
+        $download = new DownloadImages($client, sys_get_temp_dir().'/wallabag_test', 'http://wallabag.io/', $logger);
+
+        $res = $download->processSingleImage(
+            123,
+            'https://cdn.theconversation.com/files/157200/article/width926/gsj2rjp2-1487348607.jpg',
+            'https://theconversation.com/conversation-avec-gerald-bronner-ce-nest-pas-la-post-verite-qui-nous-menace-mais-lextension-de-notre-credulite-73089'
+        );
+
+        $this->assertContains('http://wallabag.io/assets/images/9/b/9b0ead26/', $res, 'Content-Type was empty but data is ok for an image');
+        $this->assertContains('DownloadImages: Checking extension (alternative)', $logHandler->getRecords()[3]['message']);
+    }
 }