diff --git a/build/psalm-baseline.xml b/build/psalm-baseline.xml
index be0729a3929d0..3f1c59d91a83a 100644
--- a/build/psalm-baseline.xml
+++ b/build/psalm-baseline.xml
@@ -1648,11 +1648,6 @@
       <code><![CDATA[$this->timeFactory->getTime()]]></code>
     </InvalidScalarArgument>
   </file>
-  <file src="core/Command/Maintenance/Mimetype/GenerateMimetypeFileBuilder.php">
-    <InvalidArrayAccess>
-      <code><![CDATA[$key[0]]]></code>
-    </InvalidArrayAccess>
-  </file>
   <file src="core/Command/Maintenance/Mimetype/UpdateDB.php">
     <UndefinedInterfaceMethod>
       <code><![CDATA[getAllMappings]]></code>
diff --git a/core/Command/Maintenance/Mimetype/GenerateMimetypeFileBuilder.php b/core/Command/Maintenance/Mimetype/GenerateMimetypeFileBuilder.php
index 873744e6f9480..db2f3e44f1cdf 100644
--- a/core/Command/Maintenance/Mimetype/GenerateMimetypeFileBuilder.php
+++ b/core/Command/Maintenance/Mimetype/GenerateMimetypeFileBuilder.php
@@ -37,6 +37,10 @@ class GenerateMimetypeFileBuilder {
 	public function generateFile(array $aliases): string {
 		// Remove comments
 		$aliases = array_filter($aliases, static function ($key) {
+			// Single digit extensions will be treated as integers
+			// Let's make sure they are strings
+			// https://github.com/nextcloud/server/issues/42902
+			$key = (string)$key;
 			return !($key === '' || $key[0] === '_');
 		}, ARRAY_FILTER_USE_KEY);
 
diff --git a/core/Command/Maintenance/Mimetype/UpdateDB.php b/core/Command/Maintenance/Mimetype/UpdateDB.php
index 212b1994263c0..26c0b3aba6ea6 100644
--- a/core/Command/Maintenance/Mimetype/UpdateDB.php
+++ b/core/Command/Maintenance/Mimetype/UpdateDB.php
@@ -62,6 +62,10 @@ protected function execute(InputInterface $input, OutputInterface $output): int
 		$totalNewMimetypes = 0;
 
 		foreach ($mappings as $ext => $mimetypes) {
+			// Single digit extensions will be treated as integers
+			// Let's make sure they are strings
+			// https://github.com/nextcloud/server/issues/42902
+			$ext = (string)$ext;
 			if ($ext[0] === '_') {
 				// comment
 				continue;
diff --git a/lib/private/Files/Type/Detection.php b/lib/private/Files/Type/Detection.php
index 71b8cb986d71a..05901a6f0b939 100644
--- a/lib/private/Files/Type/Detection.php
+++ b/lib/private/Files/Type/Detection.php
@@ -98,6 +98,8 @@ public function __construct(IURLGenerator $urlGenerator,
 	public function registerType(string $extension,
 		string $mimetype,
 		?string $secureMimeType = null): void {
+		// Make sure the extension is a string
+		// https://github.com/nextcloud/server/issues/42902
 		$this->mimetypes[$extension] = [$mimetype, $secureMimeType];
 		$this->secureMimeTypes[$mimetype] = $secureMimeType ?: $mimetype;
 	}
@@ -112,13 +114,17 @@ public function registerType(string $extension,
 	 * @param array $types
 	 */
 	public function registerTypeArray(array $types): void {
-		$this->mimetypes = array_merge($this->mimetypes, $types);
+		// Register the types,
+		foreach ($types as $extension => $mimeType) {
+			$this->registerType((string)$extension, $mimeType[0], $mimeType[1] ?? null);
+		}
 
 		// Update the alternative mimetypes to avoid having to look them up each time.
 		foreach ($this->mimetypes as $extension => $mimeType) {
-			if (str_starts_with($extension, '_comment')) {
+			if (str_starts_with((string)$extension, '_comment')) {
 				continue;
 			}
+
 			$this->secureMimeTypes[$mimeType[0]] = $mimeType[1] ?? $mimeType[0];
 			if (isset($mimeType[1])) {
 				$this->secureMimeTypes[$mimeType[1]] = $mimeType[1];
@@ -179,7 +185,7 @@ private function loadMappings(): void {
 	}
 
 	/**
-	 * @return array
+	 * @return array<string, list{string, string|null}>
 	 */
 	public function getAllMappings(): array {
 		$this->loadMappings();
@@ -209,7 +215,7 @@ public function detectPath($path): string {
 			$extension = strrchr($fileName, '.');
 			if ($extension !== false) {
 				$extension = strtolower($extension);
-				$extension = substr($extension, 1); //remove leading .
+				$extension = substr($extension, 1); // remove leading .
 				return $this->mimetypes[$extension][0] ?? 'application/octet-stream';
 			}
 		}
diff --git a/tests/lib/Files/Type/DetectionTest.php b/tests/lib/Files/Type/DetectionTest.php
index 079117f562257..37bab1a46c0d2 100644
--- a/tests/lib/Files/Type/DetectionTest.php
+++ b/tests/lib/Files/Type/DetectionTest.php
@@ -114,6 +114,41 @@ public function testDetectString(): void {
 		$this->assertEquals($expected, $result);
 	}
 
+	public function dataMimeTypeCustom(): array {
+		return [
+			['123', 'foobar/123'],
+			['a123', 'foobar/123'],
+			['bar', 'foobar/bar'],
+		];
+	}
+
+	/**
+	 * @dataProvider dataMimeTypeCustom
+	 *
+	 * @param string $ext
+	 * @param string $mime
+	 */
+	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]]));
+
+		$detection = new Detection($urlGenerator, $logger, $confDir, $confDir);
+		$mappings = $detection->getAllMappings();
+		$this->assertArrayHasKey($ext, $mappings);
+		$this->assertEquals($mime, $detection->detectPath('foo.' . $ext));
+	}
+
 	public function dataGetSecureMimeType(): array {
 		return [
 			['image/svg+xml', 'text/plain'],