diff --git a/src/wp-includes/functions.php b/src/wp-includes/functions.php
index f5002a45de1e8..e386a23ccf12d 100644
--- a/src/wp-includes/functions.php
+++ b/src/wp-includes/functions.php
@@ -7494,6 +7494,7 @@ function _device_can_upload() {
  * Tests if a given path is a stream URL
  *
  * @since 3.5.0
+ * @since 7.2.0 A lowercased scheme is accepted as a fallback, matching PHP.
  *
  * @param string $path The resource path or URL.
  * @return bool True if the path is a stream URL.
@@ -7506,9 +7507,15 @@ function wp_is_stream( $path ) {
 		return false;
 	}
 
-	$stream = substr( $path, 0, $scheme_separator );
+	$stream   = substr( $path, 0, $scheme_separator );
+	$wrappers = stream_get_wrappers();
 
-	return in_array( $stream, stream_get_wrappers(), true );
+	/*
+	 * PHP looks the scheme up as given, then retries once with it lowercased.
+	 * This is not a case-insensitive match: a wrapper registered as `MyStream`
+	 * is not reachable as `mystream`.
+	 */
+	return in_array( $stream, $wrappers, true ) || in_array( strtolower( $stream ), $wrappers, true );
 }
 
 /**
diff --git a/tests/phpunit/tests/functions.php b/tests/phpunit/tests/functions.php
index 19c721aeaa46a..9db34df8f4a87 100644
--- a/tests/phpunit/tests/functions.php
+++ b/tests/phpunit/tests/functions.php
@@ -2007,6 +2007,8 @@ public function data_validate_file() {
 	/**
 	 * Test stream URL validation.
 	 *
+	 * @ticket 65870
+	 *
 	 * @dataProvider data_wp_is_stream
 	 *
 	 * @param string $path     The resource path or URL.
@@ -2037,6 +2039,7 @@ public function data_wp_is_stream() {
 			array( 'https://example.com', true ),
 			array( 'ftp://example.com', true ),
 			array( 'file:///path/to/some/file', true ),
+			array( 'FILE:///path/to/some/file', true ),
 			array( 'php://some/php/file.php', true ),
 
 			// Non-stream examples.
@@ -2047,6 +2050,25 @@ public function data_wp_is_stream() {
 		);
 	}
 
+	/**
+	 * Tests that a wrapper registered under a mixed case scheme is not
+	 * reachable under a lowercased scheme.
+	 *
+	 * @ticket 65870
+	 */
+	public function test_wp_is_stream_does_not_lowercase_registered_wrappers() {
+		require_once DIR_TESTROOT . '/includes/class-wp-test-stream.php';
+		stream_wrapper_register( 'wpTestMixedCase', 'WP_Test_Stream' );
+
+		$exact      = wp_is_stream( 'wpTestMixedCase://foo' );
+		$lowercased = wp_is_stream( 'wptestmixedcase://foo' );
+
+		stream_wrapper_unregister( 'wpTestMixedCase' );
+
+		$this->assertTrue( $exact, 'The scheme was not matched as registered.' );
+		$this->assertFalse( $lowercased, 'A lowercased scheme matched a mixed case wrapper.' );
+	}
+
 	/**
 	 * Test human_readable_duration().
 	 *