diff --git a/lib/private/Files/Type/Detection.php b/lib/private/Files/Type/Detection.php
index 6af6ce1a0b1e4..2fa2db71ddcd0 100644
--- a/lib/private/Files/Type/Detection.php
+++ b/lib/private/Files/Type/Detection.php
@@ -97,7 +97,7 @@ private function loadCustomDefinitions(string $fileName, array $definitions): ar
 		if (file_exists($this->customConfigDir . '/' . $fileName)) {
 			$custom = json_decode(file_get_contents($this->customConfigDir . '/' . $fileName), true);
 			if (json_last_error() === JSON_ERROR_NONE) {
-				$definitions = array_merge($definitions, $custom);
+				$definitions = array_replace($definitions, $custom);
 			} else {
 				$this->logger->warning('Failed to parse ' . $fileName . ': ' . json_last_error_msg());
 			}
diff --git a/tests/lib/Files/Type/DetectionTest.php b/tests/lib/Files/Type/DetectionTest.php
index c30b21de401d2..6e4a4f4c70c21 100644
--- a/tests/lib/Files/Type/DetectionTest.php
+++ b/tests/lib/Files/Type/DetectionTest.php
@@ -117,24 +117,72 @@ public static function dataMimeTypeCustom(): array {
 	 */
 	#[\PHPUnit\Framework\Attributes\DataProvider('dataMimeTypeCustom')]
 	public function testDetectMimeTypeCustom(string $ext, string $mime): void {
-		$confDir = sys_get_temp_dir();
-		file_put_contents($confDir . '/mimetypemapping.dist.json', json_encode([]));
-
-		/** @var IURLGenerator $urlGenerator */
-		$urlGenerator = $this->getMockBuilder(IURLGenerator::class)
-			->disableOriginalConstructor()
-			->getMock();
-
-		/** @var LoggerInterface $logger */
-		$logger = $this->createMock(LoggerInterface::class);
-
-		// Create new mapping file
-		file_put_contents($confDir . '/mimetypemapping.dist.json', json_encode([$ext => [$mime]]));
+		$tmpDir = sys_get_temp_dir() . '/nc-detect-' . uniqid('', true);
+		mkdir($tmpDir, 0700);
+
+		try {
+			// Create a default / shipped mapping file (dist)
+			file_put_contents($tmpDir . '/mimetypemapping.dist.json', json_encode([]));
+
+			// Create new custom mapping file that should be picked up by Detection
+			file_put_contents($tmpDir . '/mimetypemapping.json', json_encode([$ext => [$mime]]));
+
+			/** @var IURLGenerator $urlGenerator */
+			$urlGenerator = $this->getMockBuilder(IURLGenerator::class)
+				->disableOriginalConstructor()
+				->getMock();
+
+			/** @var LoggerInterface $logger */
+			$logger = $this->createMock(LoggerInterface::class);
+
+			$detection = new Detection($urlGenerator, $logger, $tmpDir, $tmpDir);
+			$mappings = $detection->getAllMappings();
+
+			$this->assertArrayHasKey($ext, $mappings);
+			$this->assertEquals($mime, $detection->detectPath('foo.' . $ext));
+		} finally {
+			// cleanup
+			@unlink($tmpDir . '/mimetypemapping.json');
+			@unlink($tmpDir . '/mimetypemapping.dist.json');
+			@rmdir($tmpDir);
+		}
+	}
 
-		$detection = new Detection($urlGenerator, $logger, $confDir, $confDir);
-		$mappings = $detection->getAllMappings();
-		$this->assertArrayHasKey($ext, $mappings);
-		$this->assertEquals($mime, $detection->detectPath('foo.' . $ext));
+	public function testDetectMimeTypePreservesLeadingZeroKeys(): void {
+		$tmpDir = sys_get_temp_dir() . '/nc-detect-' . uniqid();
+		mkdir($tmpDir, 0700);
+		try {
+			// Create a default / shipped mapping file (dist)
+			file_put_contents($tmpDir . '/mimetypemapping.dist.json', json_encode([]));
+
+			// Create new custom mapping file with potentially problematic keys
+			$mappings = [
+				'001' => ['application/x-zeroone', null],
+				'1' => ['application/x-one', null],
+			];
+			file_put_contents($tmpDir . '/mimetypemapping.json', json_encode($mappings));
+
+			/** @var IURLGenerator $urlGenerator */
+			$urlGenerator = $this->getMockBuilder(IURLGenerator::class)
+				->disableOriginalConstructor()
+				->getMock();
+
+			/** @var LoggerInterface $logger */
+			$logger = $this->createMock(LoggerInterface::class);
+
+			$detection = new Detection($urlGenerator, $logger, $tmpDir, $tmpDir);
+			$mappings = $detection->getAllMappings();
+
+			$this->assertArrayHasKey('001', $mappings, 'Expected mapping to contain key "001"');
+			$this->assertArrayHasKey('1', $mappings, 'Expected mapping to contain key "1"');
+
+			$this->assertEquals('application/x-zeroone', $detection->detectPath('foo.001'));
+			$this->assertEquals('application/x-one', $detection->detectPath('foo.1'));
+		} finally {
+			@unlink($tmpDir . '/mimetypemapping.json');
+			@unlink($tmpDir . '/mimetypemapping.dist.json');
+			@rmdir($tmpDir);
+		}
 	}
 
 	public static function dataGetSecureMimeType(): array {