ModelPostProcessor.cs

using System;
using UnityEditor;

//NOTE: Must be placed under a folder named "Editor" as it is a Unity editor script

public class ModelPostProcessor : AssetPostprocessor
{

    static readonly string[] supportedFileExtensions = new string[] { ".fbx", ".blend" };

    //-------------------------------------------------------------------------------------- EASIEST WAY
    public void OnPreprocessModel()
    {
        ModelImporter importer = assetImporter as ModelImporter;
        String name = importer.assetPath.ToLower();

        if (IsSupportedExtension(assetPath))
        {
            //Standard unity way
            importer.materialImportMode = ModelImporterMaterialImportMode.ImportStandard;
            importer.materialLocation = ModelImporterMaterialLocation.External;
            importer.materialName = ModelImporterMaterialName.BasedOnMaterialName;
            importer.materialSearch = ModelImporterMaterialSearch.Everywhere;

            //Alternate way
            //importer.materialImportMode = ModelImporterMaterialImportMode.None;
        }
    }

    private bool IsSupportedExtension(string assetPath)
    {
        for (int i = 0; i < supportedFileExtensions.Length; i++)
        {
            if (assetPath.EndsWith(supportedFileExtensions[i]))
            {
                return true;
            }
        }
        return false;
    }

    //-------------------------------------------------------------------------------------- ALTERNATE WAY
    /*
    static readonly string[] materialLocations = new string[] { "Assets/Project/Materials/" };

    // Alternative way of looking at specific folders
    public Material OnAssignMaterialModel(Material material, Renderer renderer)
    {
        for (int i = 0; i < materialLocations.Length; i++)
        {
            //Local path (For AssetDatabase use)
            string matPath = Path.Combine(materialLocations[i], material.name + ".mat");
            //Disk path (for System.IO use)
            string drivePath = matPath.Remove(matPath.IndexOf("Assets/"), "Assets/".Length);
            drivePath = Path.Combine(Application.dataPath, drivePath);

            Log("Trying to find " + material.name + " at " + drivePath);

            bool assetExists = File.Exists(drivePath);
            if (assetExists)
            {
                if (AssetDatabase.LoadAssetAtPath(matPath, typeof(Material)))
                    return AssetDatabase.LoadAssetAtPath(matPath, typeof(Material)) as Material;
            }
        }
        Log("Could not find material!!!", "red");
        return AssetDatabase.LoadAssetAtPath("Assets/Project/Materials/Default.mat", typeof(Material)) as Material;
    }

    void Log(string text, string color = "grey")
    {
        Debug.Log(string.Format("<color={1}>[Material Processing] {0}</color>", text, color));
    }
    */
}