PBXProject.IsKnownExtension

Проверяет, известны ли файлы с данным расширением PBXProject.

using UnityEngine;
using UnityEditor;
using UnityEditor.Callbacks;
using UnityEditor.iOS.Xcode;

public class Sample_IsKnownExtension  
{
    [PostProcessBuild]
    public static void OnPostprocessBuild(BuildTarget buildTarget, string pathToBuiltProject)
    {
        // Specify file extension you want to check
        string extension = "txt";

        // Check if the file extension is known to PBXProject
        bool isKnown = PBXProject.IsKnownExtension(extension);
        Debug.Log("Extension " + extension + " " + (isKnown ? "is" : "is not") + " known to PBXProject");
    }
}