import jenkins.model.Jenkins
import com.cloudbees.plugins.credentials.CredentialsProvider
import com.cloudbees.plugins.credentials.domains.DomainRequirement
import org.jenkinsci.plugins.plaincredentials.FileCredentials

def jenkins = Jenkins.getInstance()

// ID of your file credential
def credentialsId = 'credentials ID'

def lookupCredentials = CredentialsProvider.lookupCredentials(
    FileCredentials.class,
    jenkins,
    null,
    (DomainRequirement) null
)

def fileCredential = lookupCredentials.find { it.id == credentialsId }

if (fileCredential) {
    def inputStream = fileCredential.content
    def fileContent = inputStream.text
    inputStream.close()
    println("File content: \n$fileContent")
} else {
    println("Credential with ID '$credentialsId' not found")
}