Can't read configuration in other file.

[takanorig]

Hi,
I'm using this plugin for the first time.

When I defile custom configuration in one file, this plugin can read them.

• serverless.yml

custom:
    greengrass:
        groupId: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
        autoDeploy: false

functions:
    my_func:
        handler:my_func.function_handler
        greengrass:
            pinned: true

But, when I define custom configuration in other file, this plugin can't read them.

• serverless.yml

custom: ${file(../customs/${opt:stage, 'dev'}.yml)}

functions:
    my_func:
        handler:my_func.function_handler
        greengrass:
            pinned: true

• customs/dev.yml

greengrass:
    groupId: xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx
    autoDeploy: false

it showed the following debug message.

No Group ID defined, skipping..

Can it be defined the configuration in other file?