diff --git a/lib/Controller/AssistantApiController.php b/lib/Controller/AssistantApiController.php
index 6f0270721..1b9ff145b 100644
--- a/lib/Controller/AssistantApiController.php
+++ b/lib/Controller/AssistantApiController.php
@@ -399,10 +399,18 @@ public function getOutputFilePreview(int $ocpTaskId, int $fileId, ?int $x = 100,
 	public function getOutputFile(int $ocpTaskId, int $fileId): DataDownloadResponse|DataResponse {
 		try {
 			$taskOutputFile = $this->assistantService->getTaskOutputFile($this->userId, $ocpTaskId, $fileId);
-			$realMime = mime_content_type($taskOutputFile->fopen('rb'));
+			$head = fread($taskOutputFile->fopen('rb'), 4096);
+			$realMime = (new \finfo(FILEINFO_MIME_TYPE))->buffer($head);
+
+			$fileName = $ocpTaskId . '-' . $fileId;
+			$extension = pathinfo($taskOutputFile->getName(), PATHINFO_EXTENSION);
+			if ($extension !== '') {
+				$fileName .= '.' . $extension;
+			}
+
 			$response = new DataDownloadResponse(
 				$taskOutputFile->getContent(),
-				$ocpTaskId . '-' . $fileId,
+				$fileName,
 				$realMime ?: 'application/octet-stream',
 			);
 			$response->cacheFor(60 * 60 * 24, false, true);
diff --git a/lib/Service/AssistantService.php b/lib/Service/AssistantService.php
index 7376546e1..be045b45b 100644
--- a/lib/Service/AssistantService.php
+++ b/lib/Service/AssistantService.php
@@ -628,7 +628,8 @@ public function saveOutputFile(string $userId, int $ocpTaskId, int $fileId): arr
 	 * @throws NotPermittedException
 	 */
 	private function getTargetFileName(File $file): string {
-		$mimeType = mime_content_type($file->fopen('rb'));
+		$head = fread($file->fopen('rb'), 4096);
+		$mimeType = (new \finfo(FILEINFO_MIME_TYPE))->buffer($head);
 		$fileName = $file->getName();
 
 		$mimes = new \Mimey\MimeTypes;
@@ -692,7 +693,8 @@ private function extractFileIdsFromTask(Task $task): array {
 	 */
 	public function getOutputFilePreviewFile(string $userId, int $taskId, int $fileId, ?int $x = 100, ?int $y = 100): ?array {
 		$taskOutputFile = $this->getTaskOutputFile($userId, $taskId, $fileId);
-		$realMime = mime_content_type($taskOutputFile->fopen('rb'));
+		$head = fread($taskOutputFile->fopen('rb'), 4096);
+		$realMime = (new \finfo(FILEINFO_MIME_TYPE))->buffer($head);
 		return $this->previewService->getFilePreviewFile($taskOutputFile, $x, $y, $realMime ?: null);
 	}