Thanks a lot for your help.
I have a Samsung Galaxy S7 Edge "Duos" and since the beginning, I have been extremely pleased with the battery duration.
Until today...
Same

problem: "Media server" draining 53% of the battery, out of the blue...
While reading your comments, I remembered that when I turned the phone on this morning, the theme I was using (it's called "Charisma", has a picture of a black panther) did flicker a lot and didn't seem stable. The phone got very hot during hours and battery got almost drained by 2:00PM.
So I just chose another theme. and everything came back to normal.
I hope that this experience will help some of you
.........................
OK, so, 24 hours later, the infamous "mediaserver" is still draining my battery. No longer at a 53% rate, but at a more moderate (though unacceptable) 23% rate.
So I went along with Plan B, took out the SDXC card (SanDisk 128GB Extreme MicroSDXC UHS-I), did a complete back up, erased the card and then formatted it with the phone's format utility, and restored all the files on the card.
.........................
24 more hours later, no signal of "mediaserver" draining the battery. Yeah!