There’s an ongoing bug in the Windows Insider Preview builds since version 18267 that is causing the File Explorer to freeze when interacting with a .mp4 file. The bug is present on all recent Insider builds except build 18282 and 18298.
As per user reports, MP4 files are crashing the File Explorer even when you only right-click, hover over, or open a folder containing an MP4 file. Opening the file, of course, freezes the File Explorer for some time, then it crashes and resets itself. After which the PC gets back to normal.
The issue is present in Insider Preview builds 18267, 18272, 18277, 18290, 18305, 18309, 18312, and 18317. Unfortunately, there’s no fix available for the MP4 files issue in File Explorer yet.
Microsoft has acknowledged the issue and is working on a fix. The company did fix the issue in build 18282 once, then again in 18298 but with the recent 183 series of Insider builds, more Windows Insider users are reporting issues with MP4 files in File Explorer.