fix(assets): recognise m4v and mkv as video extensions (#12088)

## Summary

`.m4v` and `.mkv` files render as a generic file icon in the assets
sidebar instead of a video preview because `VIDEO_EXTENSIONS` doesn't
list them, even though both formats are widely produced by ComfyUI
custom nodes and are browser-playable when written with common codecs.

## Changes

- **What**: Add `m4v` and `mkv` to `VIDEO_EXTENSIONS` in
`packages/shared-frontend-utils/src/formatUtil.ts` and extend the
existing test cases. Aligns with ComfyUI core's canonical video
extension list
(`tests-unit/folder_paths_test/filter_by_content_types_test.py:13`),
which includes both. The frontend's format registry at
`src/platform/workflow/core/types/formats.ts` also lists `.m4v` with
mime `video/x-m4v` — `formatUtil.ts` was the inconsistent surface.
- **Breaking**: None.
- **Dependencies**: None.

## Review Focus

`m4v` is Apple's MP4 container variant; `mkv` is the Matroska container.
ComfyUI custom nodes most commonly produce both with H.264/VP9 codecs,
which play natively in modern browsers via `<video>`. Adding the
extensions routes those files through the existing `MediaVideoTop`
component without any new rendering logic.

If a user's `.mkv` happens to use an exotic codec the browser can't play
(e.g. H.265/HEVC in Chrome), they get the same controllable failure mode
as today's `.avi` entries — a `<video>` element with the browser's
native unsupported-source UI. That is no worse than the current "show a
generic file icon" behavior, and strictly better in the common case.

## Screenshots (if applicable)

> **Note**: Screenshots taken from the OSS *input* assets sidebar with
[#12086](https://github.com/Comfy-Org/ComfyUI_frontend/pull/12086) also
applied locally. That PR fixes a separate regression where OSS input
filenames carry a `[input]` annotation suffix that breaks all
extension-based media detection — without it, `m4v`/`mkv` files (and
every other file in that sidebar) still render as the generic icon. This
PR alone is sufficient for cloud assets and OSS output history; the
input-sidebar previews require both PRs.

Before:
<img width="1197" height="714" alt="2026-05-09-031123_hyprshot"
src="https://github.com/user-attachments/assets/5c6ebc2d-aac2-411f-a2e4-51a111033184"
/>

After:
<img width="1042" height="723" alt="2026-05-09-031005_hyprshot"
src="https://github.com/user-attachments/assets/f0acc2cf-8571-4fd0-b0cd-2b8b87ff9b74"
/>

packages/shared-frontend-utils/src/formatUtil.test.ts

@@ -85,9 +85,11 @@ describe('formatUtil', () => {
     describe('video files', () => {
       it('should identify video extensions correctly', () => {
         expect(getMediaTypeFromFilename('video.mp4')).toBe('video')
+        expect(getMediaTypeFromFilename('apple.m4v')).toBe('video')
         expect(getMediaTypeFromFilename('clip.webm')).toBe('video')
         expect(getMediaTypeFromFilename('movie.mov')).toBe('video')
         expect(getMediaTypeFromFilename('film.avi')).toBe('video')
+        expect(getMediaTypeFromFilename('episode.mkv')).toBe('video')
       })
     })
 

packages/shared-frontend-utils/src/formatUtil.ts

@@ -581,7 +581,7 @@ const IMAGE_EXTENSIONS = [
   'tiff',
   'svg'
 ] as const
-const VIDEO_EXTENSIONS = ['mp4', 'webm', 'mov', 'avi'] as const
+const VIDEO_EXTENSIONS = ['mp4', 'm4v', 'webm', 'mov', 'avi', 'mkv'] as const
 const AUDIO_EXTENSIONS = ['mp3', 'wav', 'ogg', 'flac'] as const
 const THREE_D_EXTENSIONS = ['obj', 'fbx', 'gltf', 'glb', 'usdz'] as const
 const TEXT_EXTENSIONS = [