PsiFileFactory Not Generating File Extension Follow

Hello, I'm currently developing a plugin that generates a Kotlin file. Everything works except that the resulting file is for example only "TestData", and not "TestData.kt" and as such is not detected by IntelliJ IDEA as a Kotlin file. Here are the pertinent snippets:

class KotlinFileType : FileType {
    override fun getName(): String {
        return "Kotlin file"
    }

    override fun getDescription(): String {
        return "Kotlin source file"
    }

    override fun getDefaultExtension(): String {
        return ".kt"
    }

    override fun getIcon(): Icon? {
        return null
    }

    override fun isBinary(): Boolean {
        return false
    }

    companion object {
        val INSTANCE = KotlinFileType()
    }
}

val fileName = "TestData"

// fileContent generated elsewhere
val classFile = factory.createFileFromText(fileName, KotlinFileType.INSTANCE, fileContent)

WriteCommandAction.runWriteCommandAction(directory.project) {
 directory.add(classFile)
}

Did I miss anything? Any help would be appreciated, thanks!