diff --git a/src/Api/Encoder.php b/src/Api/Encoder.php
index 95ff7ff..420ced9 100644
--- a/src/Api/Encoder.php
+++ b/src/Api/Encoder.php
@@ -110,9 +110,8 @@ public function getFormat(ImageInterface $image): string
     {
         try {
             $mediaType = $this->getMediaType($image);
-            $extensions = $mediaType->format()->fileExtensions();
 
-            return reset($extensions)->value; // @phpstan-ignore-line
+            return array_search($mediaType->value, self::supportedFormats(), true) ?: 'jpg';
         } catch (\Exception) {
             return 'jpg';
         }
diff --git a/tests/Api/ApiTest.php b/tests/Api/ApiTest.php
index be06f9a..a95f6f9 100644
--- a/tests/Api/ApiTest.php
+++ b/tests/Api/ApiTest.php
@@ -14,7 +14,7 @@
 
 class ApiTest extends TestCase
 {
-    private $api;
+    private Api $api;
 
     public function setUp(): void
     {
@@ -26,30 +26,30 @@ public function tearDown(): void
         \Mockery::close();
     }
 
-    public function testCreateInstance()
+    public function testCreateInstance(): void
     {
         $this->assertInstanceOf(Api::class, $this->api);
     }
 
-    public function testSetImageManager()
+    public function testSetImageManager(): void
     {
         $this->api->setImageManager(ImageManager::gd());
         $this->assertInstanceOf(ImageManager::class, $this->api->getImageManager());
     }
 
-    public function testGetImageManager()
+    public function testGetImageManager(): void
     {
         $this->assertInstanceOf(ImageManager::class, $this->api->getImageManager());
     }
 
-    public function testSetManipulators()
+    public function testSetManipulators(): void
     {
         $this->api->setManipulators([\Mockery::mock(ManipulatorInterface::class)]);
         $manipulators = $this->api->getManipulators();
         $this->assertInstanceOf(ManipulatorInterface::class, $manipulators[0]);
     }
 
-    public function testSetInvalidManipulator()
+    public function testSetInvalidManipulator(): void
     {
         $this->expectException(\InvalidArgumentException::class);
         $this->expectExceptionMessage('Not a valid manipulator.');
@@ -57,12 +57,12 @@ public function testSetInvalidManipulator()
         $this->api->setManipulators([new \stdClass()]);
     }
 
-    public function testGetManipulators()
+    public function testGetManipulators(): void
     {
         $this->assertEquals([], $this->api->getManipulators());
     }
 
-    public function testRun()
+    public function testRun(): void
     {
         $image = \Mockery::mock(ImageInterface::class, function ($mock) {
             $mock->shouldReceive('origin')->andReturn(\Mockery::mock(Origin::class, function ($mock) {
@@ -88,7 +88,7 @@ public function testRun()
         $api = new Api($manager, [$manipulator]);
 
         $this->assertEquals('encoded', $api->run(
-            file_get_contents(dirname(__FILE__, 2).'/files/red-pixel.png'),
+            (string) file_get_contents(dirname(__FILE__, 2).'/files/red-pixel.png'),
             []
         ));
     }
diff --git a/tests/Api/EncoderTest.php b/tests/Api/EncoderTest.php
index 492caa7..78b9e8e 100644
--- a/tests/Api/EncoderTest.php
+++ b/tests/Api/EncoderTest.php
@@ -130,12 +130,14 @@ public function testGetFormat(): void
         $this->assertSame('png', $this->encoder->setParams(['fm' => 'png'])->getFormat($this->getImageByMimeType('image/jpeg')));
         $this->assertSame('gif', $this->encoder->setParams(['fm' => 'gif'])->getFormat($this->getImageByMimeType('image/jpeg')));
         $this->assertSame('bmp', $this->encoder->setParams(['fm' => 'bmp'])->getFormat($this->getImageByMimeType('image/jpeg')));
+        $this->assertSame('pjpg', $this->encoder->setParams(['fm' => 'pjpg'])->getFormat($this->getImageByMimeType('image/jpeg')));
 
         // Make sure we keep the current format if no format is provided
         $this->assertSame('jpg', $this->encoder->setParams(['fm' => null])->getFormat($this->getImageByMimeType('image/jpeg')));
         $this->assertSame('png', $this->encoder->setParams(['fm' => null])->getFormat($this->getImageByMimeType('image/png')));
         $this->assertSame('gif', $this->encoder->setParams(['fm' => null])->getFormat($this->getImageByMimeType('image/gif')));
         $this->assertSame('bmp', $this->encoder->setParams(['fm' => null])->getFormat($this->getImageByMimeType('image/bmp')));
+        $this->assertSame('jpg', $this->encoder->setParams(['fm' => 'null'])->getFormat($this->getImageByMimeType('image/pjpeg')));
 
         $this->assertSame('jpg', $this->encoder->setParams(['fm' => ''])->getFormat($this->getImageByMimeType('image/jpeg')));
         $this->assertSame('png', $this->encoder->setParams(['fm' => ''])->getFormat($this->getImageByMimeType('image/png')));