diff --git a/lib/Controller/AssistantApiController.php b/lib/Controller/AssistantApiController.php
index 7d876129a..6ac2e090f 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);