diff --git a/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php b/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
index 44600369df..b0cee6c148 100644
--- a/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
+++ b/core/modules/file/src/Plugin/rest/resource/FileUploadResource.php
@@ -374,7 +374,7 @@ protected function validateAndParseContentDispositionHeader(Request $request) {
 
     // Make sure only the filename component is returned. Path information is
     // stripped as per https://tools.ietf.org/html/rfc6266#section-4.3.
-    return basename($filename);
+    return $this->fileSystem->basename($filename);
   }
 
   /**
diff --git a/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php b/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
index cb3ef3e27a..994a4c3ccb 100644
--- a/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
+++ b/core/modules/rest/tests/src/Functional/FileUploadResourceTestBase.php
@@ -373,11 +373,13 @@ public function testFileUploadUnicodeFilename() {
 
     $uri = Url::fromUri('base:' . static::$postUri);
 
-    $response = $this->fileRequest($uri, $this->testFileData, ['Content-Disposition' => 'file; filename="example-✓.txt"']);
+    // It is important that the filename starts with a unicode character. See
+    // https://bugs.php.net/bug.php?id=77239.
+    $response = $this->fileRequest($uri, $this->testFileData, ['Content-Disposition' => 'file; filename="Èxample-✓.txt"']);
     $this->assertSame(201, $response->getStatusCode());
-    $expected = $this->getExpectedNormalizedEntity(1, 'example-✓.txt', TRUE);
+    $expected = $this->getExpectedNormalizedEntity(1, 'Èxample-✓.txt', TRUE);
     $this->assertResponseData($expected, $response);
-    $this->assertSame($this->testFileData, file_get_contents('public://foobar/example-✓.txt'));
+    $this->assertSame($this->testFileData, file_get_contents('public://foobar/Èxample-✓.txt'));
   }
 
   /**