I went through the schema and configured file credential. However i could see the exception below.

  - file:
      scope: GLOBAL
      id: jenkins-agent-ssh-key-file
      description: "Key to login into docker as jenkins user."
      filename: "jenkins-agent-ssh-key-file"
      secretbytes: ABC

java.lang.IllegalArgumentException: file is required to configure class org.jenkinsci.plugins.plaincredentials.impl.FileCredentialsImpl
at org.jenkinsci.plugins.casc.DataBoundConfigurator.configure(DataBoundConfigurator.java:67)
at org.jenkinsci.plugins.casc.HeteroDescribableConfigurator.configure(HeteroDescribableConfigurator.java:74)
at org.jenkinsci.plugins.casc.HeteroDescribableConfigurator.configure(HeteroDescribableConfigurator.java:30)
at org.jenkinsci.plugins.casc.credentials.CredentialsRootConfigurator.configure(CredentialsRootConfigurator.java:57)
at org.jenkinsci.plugins.casc.credentials.CredentialsRootConfigurator.configure(CredentialsRootConfigurator.java:26)
at org.jenkinsci.plugins.casc.ConfigurationAsCode.configureWith(ConfigurationAsCode.java:223)
at org.jenkinsci.plugins.casc.ConfigurationAsCode._configureWithPaths(ConfigurationAsCode.java:165)
at org.jenkinsci.plugins.casc.ConfigurationAsCode.configure(ConfigurationAsCode.java:127)
at org.jenkinsci.plugins.casc.ConfigurationAsCode.init(ConfigurationAsCode.java:110)
Caused: java.lang.reflect.InvocationTargetException