diff --git a/.phpcs.xml b/.phpcs.xml
index 4ab85427..1fc70ae8 100644
--- a/.phpcs.xml
+++ b/.phpcs.xml
@@ -11,6 +11,7 @@
     <exclude-pattern>node_modules/</exclude-pattern>
     <exclude-pattern>var/</exclude-pattern>
     <exclude-pattern>public/assets</exclude-pattern>
+    <exclude-pattern>.docker</exclude-pattern>
 
     <rule ref="SlevomatCodingStandard.Files.TypeNameMatchesFileName">
         <properties>
diff --git a/i18n/en.json b/i18n/en.json
index 58c68423..783523b2 100644
--- a/i18n/en.json
+++ b/i18n/en.json
@@ -25,6 +25,7 @@
   "translate-button": "Translate",
   "no-translations": "This file does not have any labels available for translation. Please pick another image.",
   "not-found": "The file you requested could not be found. Please pick another image.",
+  "invalid-format": "Only SVG files are supported.",
 
   "pick-another": "← pick another file",
   "view-on-commons": "View on Commons",
diff --git a/i18n/qqq.json b/i18n/qqq.json
index 5be4dfd5..d6e7cdd2 100644
--- a/i18n/qqq.json
+++ b/i18n/qqq.json
@@ -29,6 +29,7 @@
 	"translate-button": "Search button text.\n{{Identical|Translate}}",
 	"no-translations": "Error message displayed for SVGs that have nothing to translate.",
 	"not-found": "Error message displayed if the requested file could not be found.",
+	"invalid-format": "Error message displayed if the requested file is not an SVG image.",
 	"pick-another": "Label for backlink from the translate page to the search page.",
 	"view-on-commons": "Link to view the file on Commons. The whole phrase is linked.",
 	"opens-in-new-tab": "Tooltip text for links that will open in a new browser tab.",
diff --git a/public/assets/i18n/app/en.json b/public/assets/i18n/app/en.json
index 58c68423..783523b2 100644
--- a/public/assets/i18n/app/en.json
+++ b/public/assets/i18n/app/en.json
@@ -25,6 +25,7 @@
   "translate-button": "Translate",
   "no-translations": "This file does not have any labels available for translation. Please pick another image.",
   "not-found": "The file you requested could not be found. Please pick another image.",
+  "invalid-format": "Only SVG files are supported.",
 
   "pick-another": "← pick another file",
   "view-on-commons": "View on Commons",
diff --git a/public/assets/i18n/app/qqq.json b/public/assets/i18n/app/qqq.json
index 5be4dfd5..d6e7cdd2 100644
--- a/public/assets/i18n/app/qqq.json
+++ b/public/assets/i18n/app/qqq.json
@@ -29,6 +29,7 @@
 	"translate-button": "Search button text.\n{{Identical|Translate}}",
 	"no-translations": "Error message displayed for SVGs that have nothing to translate.",
 	"not-found": "Error message displayed if the requested file could not be found.",
+	"invalid-format": "Error message displayed if the requested file is not an SVG image.",
 	"pick-another": "Label for backlink from the translate page to the search page.",
 	"view-on-commons": "Link to view the file on Commons. The whole phrase is linked.",
 	"opens-in-new-tab": "Tooltip text for links that will open in a new browser tab.",
diff --git a/src/Controller/TranslateController.php b/src/Controller/TranslateController.php
index 06608f6f..0f6cf4fd 100644
--- a/src/Controller/TranslateController.php
+++ b/src/Controller/TranslateController.php
@@ -4,6 +4,7 @@
 namespace App\Controller;
 
 use App\Exception\ImageNotFoundException;
+use App\Exception\InvalidFormatException;
 use App\Model\Svg\SvgFile;
 use App\Model\Title;
 use App\OOUI\TranslationsFieldset;
@@ -55,18 +56,15 @@ public function translate(
 
         // Fetch the SVG from Commons.
         try {
+            $this->assertFileType($normalizedFilename);
             $path = $cache->getPath($filename);
         } catch (ImageNotFoundException $exception) {
-            $notFoundMessage = $this->renderView(
-                'error_message.html.twig',
-                ['msg_name' => 'not-found']
-            );
-            // Flash the message to show to the user under the search form.
-            $this->addFlash('search-errors', (string)$notFoundMessage);
-            // Also flash the failed filename so we can populate the search form.
-            $this->addFlash('search-redirect', $normalizedFilename);
-            return $this->redirectToRoute('home');
+            return $this->showError('not-found', $normalizedFilename);
+        } catch (InvalidFormatException $exception) {
+            return $this->showError('invalid-format', $normalizedFilename);
         }
+
+
         $svgFile = new SvgFile($path);
 
         // If there are no strings to translate, tell the user.
@@ -227,4 +225,39 @@ public function updownload(
             return $this->redirectToRoute('translate', ['filename' => $filename]);
         }
     }
+
+    /**
+     * Shows an error related to the selected file
+     *
+     * @param string $messageKey
+     * @param string $fileName
+     * @return Response
+     */
+    private function showError(string $messageKey, string $fileName): Response
+    {
+        $message = $this->renderView(
+            'error_message.html.twig',
+            ['msg_name' => $messageKey]
+        );
+        // Flash the message to show to the user under the search form.
+        $this->addFlash('search-errors', (string)$message);
+        // Also flash the failed filename so we can populate the search form.
+        $this->addFlash('search-redirect', $fileName);
+
+        return $this->redirectToRoute('home');
+    }
+
+    /**
+     * Throws an exception if the given filename is not of an SVG file.
+     * Exceptions are used to unify handling with other places that might encounter this problem.
+     *
+     * @param string $fileName
+     */
+    private function assertFileType(string $fileName): void
+    {
+        $extension = mb_strtolower(pathinfo($fileName, PATHINFO_EXTENSION));
+        if ('svg' !== $extension) {
+            throw new InvalidFormatException($extension);
+        }
+    }
 }
diff --git a/src/Exception/InvalidFormatException.php b/src/Exception/InvalidFormatException.php
new file mode 100644
index 00000000..2fe0a485
--- /dev/null
+++ b/src/Exception/InvalidFormatException.php
@@ -0,0 +1,14 @@
+<?php
+declare(strict_types = 1);
+
+namespace App\Exception;
+
+use Symfony\Component\HttpKernel\Exception\UnsupportedMediaTypeHttpException;
+
+class InvalidFormatException extends UnsupportedMediaTypeHttpException
+{
+    public function __construct(string $format)
+    {
+        parent::__construct("Files of type '$format' are not supported");
+    }
+}
diff --git a/src/Service/Retriever.php b/src/Service/Retriever.php
index 56f68976..0d834ab8 100644
--- a/src/Service/Retriever.php
+++ b/src/Service/Retriever.php
@@ -3,6 +3,7 @@
 
 namespace App\Service;
 
+use App\Exception\InvalidFormatException;
 use GuzzleHttp\Client;
 
 /**
@@ -41,6 +42,11 @@ protected function httpGet(string $url): string
     {
         $client = new Client();
         $response = $client->get($url);
+        $contentType = $response->getHeader('Content-Type');
+        $contentType = reset($contentType);
+        if ('image/svg+xml' !== $contentType) {
+            throw new InvalidFormatException($contentType);
+        }
         return $response->getBody()->getContents();
     }
 }
diff --git a/tests/Controller/TranslateControllerTest.php b/tests/Controller/TranslateControllerTest.php
index f9fa8956..aa600b91 100644
--- a/tests/Controller/TranslateControllerTest.php
+++ b/tests/Controller/TranslateControllerTest.php
@@ -19,4 +19,12 @@ public function testExists(): void
         static::assertEquals(200, $response->getStatusCode());
         static::assertContains('Test.svg', $crawler->filter('h1')->text());
     }
+
+    public function testNonSvgHandling(): void
+    {
+        $client = static::createClient();
+        $client->request('GET', '/File:Test.jpg');
+        $response = $client->getResponse();
+        static::assertEquals(302, $response->getStatusCode());
+    }
 }