diff --git a/lib/private/Preview/Generator.php b/lib/private/Preview/Generator.php
index e68120338ef0b..5fc588788f2cf 100644
--- a/lib/private/Preview/Generator.php
+++ b/lib/private/Preview/Generator.php
@@ -400,7 +400,8 @@ private function generateProviderPreview(File $file, int $width, int $height, bo
 					$previewEntry->setMax($max);
 					$previewEntry->setCropped($crop);
 					$previewEntry->setEncrypted(false);
-					$previewEntry->setMimetype($preview->dataMimeType());
+					$mimeType = $preview->dataMimeType() ?? 'image/png';
+					$previewEntry->setMimetype($mimeType);
 					$previewEntry->setEtag($file->getEtag());
 					$previewEntry->setMtime((new \DateTime())->getTimestamp());
 					return $this->savePreview($previewEntry, $preview);
@@ -554,7 +555,7 @@ private function generatePreview(
 		$previewEntry->setMax(false);
 		$previewEntry->setCropped($crop);
 		$previewEntry->setEncrypted(false);
-		$previewEntry->setMimeType($preview->dataMimeType());
+		$previewEntry->setMimeType($preview->dataMimeType() ?? 'image/png');
 		$previewEntry->setEtag($file->getEtag());
 		$previewEntry->setMtime((new \DateTime())->getTimestamp());
 		if ($cacheResult) {
diff --git a/tests/lib/Preview/GeneratorTest.php b/tests/lib/Preview/GeneratorTest.php
index f1fab3bf0a445..7c7989ee8d932 100644
--- a/tests/lib/Preview/GeneratorTest.php
+++ b/tests/lib/Preview/GeneratorTest.php
@@ -399,22 +399,67 @@ public function testNoProvider(): void {
 		$this->generator->getPreview($file, 100, 100);
 	}
 
-	private function getMockImage(int $width, int $height, string $data = '') {
+	public function testNullMimeTypePreview(): void {
+		$file = $this->getFile(42, 'image/gif');
+
+		$this->previewManager->method('isMimeSupported')
+			->with($this->equalTo('image/gif'))
+			->willReturn(true);
+
+		$maxPreview = new Preview();
+		$maxPreview->setWidth(2048);
+		$maxPreview->setHeight(2048);
+		$maxPreview->setMax(true);
+		$maxPreview->setSize(1000);
+		$maxPreview->setVersion(null);
+		$maxPreview->setMimeType('image/png');
+
+		$this->previewMapper->method('getAvailablePreviews')
+			->with($this->equalTo([42]))
+			->willReturn([42 => [
+				$maxPreview,
+			]]);
+
+		$image = $this->getMockImage(2048, 2048, 'image data', null);
+		$this->helper->method('getImage')
+			->willReturn($image);
+
+		$this->previewMapper->method('insert')
+			->willReturnCallback(function (Preview $preview): Preview {
+				$this->assertSame('image/png', $preview->getMimeType());
+				return $preview;
+			});
+
+		$this->previewMapper->method('update')
+			->willReturnCallback(fn (Preview $preview): Preview => $preview);
+
+		$this->storageFactory->method('writePreview')
+			->willReturn(1000);
+
+		$this->eventDispatcher->expects($this->once())
+			->method('dispatchTyped')
+			->with(new BeforePreviewFetchedEvent($file, 100, 100, false, IPreview::MODE_FILL, null));
+
+		$result = $this->generator->getPreview($file, 100, 100);
+		$this->assertSame('256-256.png', $result->getName());
+	}
+
+	private function getMockImage(int $width, int $height, string $data = '', ?string $mimeType = 'image/png') {
 		$image = $this->createMock(IImage::class);
 		$image->method('height')->willReturn($width);
 		$image->method('width')->willReturn($height);
 		$image->method('valid')->willReturn(true);
-		$image->method('dataMimeType')->willReturn('image/png');
+		$image->method('dataMimeType')->willReturn($mimeType);
 		$image->method('data')->willReturn($data);
 
-		$image->method('resizeCopy')->willReturnCallback(function ($size) use ($data) {
-			return $this->getMockImage($size, $size, $data);
+		$image->method('resizeCopy')->willReturnCallback(function ($size) use ($data, $mimeType) {
+			return $this->getMockImage($size, $size, $data, $mimeType);
 		});
-		$image->method('preciseResizeCopy')->willReturnCallback(function ($width, $height) use ($data) {
-			return $this->getMockImage($width, $height, $data);
+		$image->method('preciseResizeCopy')->willReturnCallback(function ($width, $height) use ($data, $mimeType) {
+			return $this->getMockImage($width, $height, $data, $mimeType);
 		});
-		$image->method('cropCopy')->willReturnCallback(function ($x, $y, $width, $height) use ($data) {
-			return $this->getMockImage($width, $height, $data);
+		$image->method('cropCopy')->willReturnCallback(function ($x, $y, $width, $height) use ($data, $mimeType) {
+			return $this->getMockImage($width, $height, $data, $mimeType);
 		});
 
 		return $image;