diff --git a/CHANGES.md b/CHANGES.md
index 7a0c177..2d344a7 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -1,3 +1,7 @@
+## Unreleased
+
+* Added `Format` constants for the media types `convert` accepts, including JPEG XL (`image/jxl`).
+
 ## 1.6.4
 * Will use POST instead of GET when retrieving result.
 * Remove deprecated `curl_close` for PHP version 8. https://www.php.net/manual/en/function.curl-close.php.
diff --git a/lib/Tinify/Format.php b/lib/Tinify/Format.php
new file mode 100644
index 0000000..3cee741
--- /dev/null
+++ b/lib/Tinify/Format.php
@@ -0,0 +1,43 @@
+<?php
+
+namespace Tinify;
+
+/**
+ * Media types that images can be converted to.
+ *
+ * Use these constants with Source::convert():
+ *
+ *     \Tinify\fromFile("input.png")
+ *         ->convert(array("type" => \Tinify\Format::JXL))
+ *         ->toFile("output.jxl");
+ *
+ * Multiple types may be supplied, in which case the API returns the smallest
+ * result:
+ *
+ *     array("type" => array(\Tinify\Format::JXL, \Tinify\Format::WEBP))
+ */
+final class Format {
+    /** WebP. */
+    const WEBP = "image/webp";
+
+    /** PNG. */
+    const PNG = "image/png";
+
+    /** JPEG. */
+    const JPEG = "image/jpeg";
+
+    /** JPEG, an alias of self::JPEG. */
+    const JPG = "image/jpg";
+
+    /** AVIF. */
+    const AVIF = "image/avif";
+
+    /** JPEG XL. */
+    const JXL = "image/jxl";
+
+    /** Wildcard, returns the smallest of the supported types. */
+    const ANY = "*/*";
+
+    private function __construct() {
+    }
+}
diff --git a/test/TinifyFormatTest.php b/test/TinifyFormatTest.php
new file mode 100644
index 0000000..090bf76
--- /dev/null
+++ b/test/TinifyFormatTest.php
@@ -0,0 +1,50 @@
+<?php
+
+use Tinify\CurlMock;
+
+class TinifyFormatTest extends TestCase {
+    public function testFormatShouldExposeSupportedMediaTypes() {
+        $this->assertSame("image/webp", Tinify\Format::WEBP);
+        $this->assertSame("image/png", Tinify\Format::PNG);
+        $this->assertSame("image/jpeg", Tinify\Format::JPEG);
+        $this->assertSame("image/jpg", Tinify\Format::JPG);
+        $this->assertSame("image/avif", Tinify\Format::AVIF);
+        $this->assertSame("image/jxl", Tinify\Format::JXL);
+        $this->assertSame("*/*", Tinify\Format::ANY);
+    }
+
+    public function testConvertWithFormatShouldSerializeMediaType() {
+        Tinify\setKey("valid");
+
+        CurlMock::register("https://api.tinify.com/shrink", array(
+            "status" => 201, "headers" => array("Location" => "https://api.tinify.com/some/location")
+        ));
+
+        CurlMock::register("https://api.tinify.com/some/location", array(
+            "status" => 200, "body" => "converted file"
+        ));
+
+        Tinify\Source::fromBuffer("png file")->convert(array("type" => Tinify\Format::JXL))->toBuffer();
+        $this->assertSame("{\"convert\":{\"type\":\"image\/jxl\"}}", CurlMock::last(CURLOPT_POSTFIELDS));
+    }
+
+    public function testConvertWithMultipleFormatsShouldSerializeMediaTypes() {
+        Tinify\setKey("valid");
+
+        CurlMock::register("https://api.tinify.com/shrink", array(
+            "status" => 201, "headers" => array("Location" => "https://api.tinify.com/some/location")
+        ));
+
+        CurlMock::register("https://api.tinify.com/some/location", array(
+            "status" => 200, "body" => "converted file"
+        ));
+
+        Tinify\Source::fromBuffer("png file")
+            ->convert(array("type" => array(Tinify\Format::JXL, Tinify\Format::WEBP)))
+            ->toBuffer();
+        $this->assertSame(
+            "{\"convert\":{\"type\":[\"image\/jxl\",\"image\/webp\"]}}",
+            CurlMock::last(CURLOPT_POSTFIELDS)
+        );
+    }
+}