diff --git a/lib/Service/AssistantService.php b/lib/Service/AssistantService.php
index ee6e3fbd..5e3dad77 100644
--- a/lib/Service/AssistantService.php
+++ b/lib/Service/AssistantService.php
@@ -743,7 +743,7 @@ 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'));
+		$mimeType = $this->detectMimeType($file->fopen('rb'));
 		$fileName = $file->getName();
 
 		$mimes = new \Mimey\MimeTypes;
@@ -755,6 +755,29 @@ private function getTargetFileName(File $file): string {
 		return $fileName;
 	}
 
+	/**
+	 * Wraps mime_content_type() to avoid a PHP warning when given a stream
+	 * that does not implement stream_cast() (e.g. Icewind\Streams\CallbackWrapper,
+	 * returned by File::fopen() for some storage backends). The stream is
+	 * drained into a temp file first so mime_content_type() can operate on a
+	 * real path instead of the unsupported stream resource.
+	 *
+	 * @param resource $stream
+	 * @return string|false
+	 */
+	private function detectMimeType($stream) {
+		$tmpFile = tempnam(sys_get_temp_dir(), 'nc_assistant_mime_');
+		$tmpHandle = fopen($tmpFile, 'wb');
+		stream_copy_to_stream($stream, $tmpHandle);
+		fclose($tmpHandle);
+		if (is_resource($stream)) {
+			fclose($stream);
+		}
+		$mimeType = mime_content_type($tmpFile);
+		unlink($tmpFile);
+		return $mimeType;
+	}
+
 	/**
 	 * @param Task $task
 	 * @return array
@@ -807,7 +830,7 @@ 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'));
+		$realMime = $this->detectMimeType($taskOutputFile->fopen('rb'));
 		return $this->previewService->getFilePreviewFile($taskOutputFile, $x, $y, $realMime ?: null);
 	}