I joined this community to improve stability of Microsoft Edge because it is my favorite browser.
Now my problem I cannot add some of Chromium extensions from File Explorer, IDM(Internet Download Manager) is the popular download manager for Windows. I tried 3 times but showing "We can't add apps, extensions or user scripts from this website" according to screenshot, even after enabled allow extensions from other source option. I am using Microsoft Edge beta Version 188.8.131.52 (Official build) beta (64-bit) in Windows 10 May Update 2019 (64 Bit).
if it fails or still doesn't work, i'd suggest using the Dev or Canary channel instead until the Beta gets its fixes and patches. I can confirm installing the extension from that link works with IDM on Edge Canary.