diff --git a/packages/backend/src/const.ts b/packages/backend/src/const.ts
index 50ccecd571..12eb6126c4 100644
--- a/packages/backend/src/const.ts
+++ b/packages/backend/src/const.ts
@@ -56,6 +56,9 @@ export const FILE_TYPE_BROWSERSAFE = [
 	'video/webm',
 	'audio/webm',
 
+	// MKV files
+	'video/x-matroska',
+
 	'audio/aac',
 
 	// see https://github.com/misskey-dev/misskey/pull/10686
diff --git a/packages/frontend-shared/js/const.ts b/packages/frontend-shared/js/const.ts
index 5bbfd314a4..9258814d97 100644
--- a/packages/frontend-shared/js/const.ts
+++ b/packages/frontend-shared/js/const.ts
@@ -40,6 +40,9 @@ export const FILE_TYPE_BROWSERSAFE = [
 	'video/webm',
 	'audio/webm',
 
+	// MKV files
+	'video/x-matroska',
+
 	'audio/aac',
 
 	// see https://github.com/misskey-dev/misskey/pull/10686