Some cameras like the Insta360one use their own file extensions rather than using the standard extension for the image format that they use (mp4 in this case).

To fix it, just rename the filenames to have .mp4 extension 

In the case of Windows OS, file extensions are hidden by default. Open a file browser and click on View->File extensions, then you can rename them. 

For that particular camera, additional support will be added in 8.8.7 final release to avoid the renaming  need, but current beta still requires to do it.