Object does not exist error importing custom file

i am importing custom yml file for provider section. i am getting below error when i execute sls deploy command. could you please some one help me on this?

in main serverless.yml

provider: ${file(custom.yml)}

custom.yml

provider:
name: aws
runtime: nodejs10.x

I am getting below error:

$ sls deploy -s dev

Serverless Error ---------------------------------------

The specified provider “[object Object]” does not exist.

Your Environment Information ---------------------------
Operating System: win32
Node Version: 10.13.0
Framework Version: 1.50.0
Plugin Version: 1.3.8
SDK Version: 2.1.0