diff --git a/packages/backend/src/core/FileInfoService.ts b/packages/backend/src/core/FileInfoService.ts
index 6250d4d3a1..2e2fa1e6f0 100644
--- a/packages/backend/src/core/FileInfoService.ts
+++ b/packages/backend/src/core/FileInfoService.ts
@@ -330,7 +330,7 @@ export class FileInfoService {
 	}
 
 	@bindThis
-	public fixMime(mime: string | fileType.MimeType): string {
+	public fixMime(mime: string): string {
 		// see https://github.com/misskey-dev/misskey/pull/10686
 		if (mime === 'audio/x-flac') {
 			return 'audio/flac';