Personally I think MediatR can add unneeded complexity to a project but when using it one thing I like to do is keep the command/query and its related handler in the same file. Perhaps not one for the purists but for me this makes it a little easier to work with.
Most devs seem to go with the approach of keeping the command/query and its handler in different files, although on Twitter the results were pretty close.
What do you like to do? Same file or different files?