A file named appears to have two video container extensions.
A flexible container that can hold unlimited video, audio, and subtitle tracks in one file. IPX-550.mkv.mp4
Drag and drop your "IPX-550.mkv.mp4" into Handbrake. Preset: Choose "Fast 1080p30" for general use. Format: Select MP4 in the "Summary" tab. A file named appears to have two video container extensions