diff --git a/src/Core/GSFile.php b/src/Core/GSFile.php
index c1733bf949..770e55dc3f 100644
--- a/src/Core/GSFile.php
+++ b/src/Core/GSFile.php
@@ -221,7 +221,7 @@ class GSFile
     /**
      * Get the minor part of a mimetype. image/webp -> image
      */
-    public static function mimetypeMajor(string $mime)
+    public static function mimetypeMajor(string $mime): string
     {
         return explode('/', self::mimetypeBare($mime))[0];
     }
@@ -229,7 +229,7 @@ class GSFile
     /**
      * Get the minor part of a mimetype. image/webp -> webp
      */
-    public static function mimetypeMinor(string $mime)
+    public static function mimetypeMinor(string $mime): string
     {
         return explode('/', self::mimetypeBare($mime))[1];
     }
@@ -237,7 +237,7 @@ class GSFile
     /**
      *  Get only the mimetype and not additional info (separated from bare mime with semi-colon)
      */
-    public static function mimetypeBare(string $mimetype)
+    public static function mimetypeBare(string $mimetype): string
     {
         $mimetype = mb_strtolower($mimetype);
         if (($semicolon = mb_strpos($mimetype, ';')) !== false) {
diff --git a/tests/Core/GSFileTest.php b/tests/Core/GSFileTest.php
new file mode 100644
index 0000000000..bb6827ce43
--- /dev/null
+++ b/tests/Core/GSFileTest.php
@@ -0,0 +1,46 @@
+<?php
+
+// {{{ License
+
+// This file is part of GNU social - https://www.gnu.org/software/social
+//
+// GNU social is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// GNU social is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with GNU social.  If not, see <http://www.gnu.org/licenses/>.
+
+// }}}
+
+namespace App\Tests\Core;
+
+use App\Core\GSFile;
+use App\Util\GNUsocialTestCase;
+
+class GSFileTest extends GNUsocialTestCase
+{
+    public function testMimetype()
+    {
+        static::assertSame('image', GSFile::mimetypeMajor('image/png'));
+        static::assertSame('image', GSFile::mimetypeMajor('IMAGE/PNG'));
+        static::assertSame('image', GSFile::mimetypeMajor('image/jpeg'));
+        static::assertSame('text', GSFile::mimetypeMajor('text/html'));
+        static::assertSame('text', GSFile::mimetypeMajor('text/html; charset=utf-8'));
+
+        static::assertSame('png', GSFile::mimetypeMinor('image/png'));
+        static::assertSame('png', GSFile::mimetypeMinor('IMAGE/PNG'));
+        static::assertSame('jpeg', GSFile::mimetypeMinor('image/jpeg'));
+        static::assertSame('html', GSFile::mimetypeMinor('text/html'));
+        static::assertSame('html', GSFile::mimetypeMinor('text/html; charset=utf-8'));
+
+        static::assertSame('text/html', GSFile::mimetypeBare('text/html'));
+        static::assertSame('text/html', GSFile::mimetypeBare('text/html; charset=utf-8'));
+    }
+}