The Display documentation specifies that you can include files with the following instructions. There is however a catch if you combine it with zipping the files.

.

Non-zipped files

Files in Magic Contest Documents\Display
a.mcy
i.mcy

Magic Contest Display setting
File name: a

File A:
...
[INCLUDE_FILE]
Content = i

This works just fine if the included file has the file extension mcy and the files are placed in the Magic Contest Documents\Display folder.

.

Zipped files

There is a catch, and that is when you zip the mcy files. This can be done for instance to show less files in the Display folder, or to target multiple projectors with different layouts.

In this case the included file must have a valid file extension (inc or incl) and the instruction must also specify this:

Files in Magic Contest Documents\Display
a.mcy
b.mcy
i.inc

Zipped file containing a.mcy, b.mcy and i.inc:
z.zip

Magic Contest Display setting
File name: z

File A:
...
[INCLUDE_FILE]
Content = i.inc