Create TexturePostProcessor.cs file anywhere in Assets folder:
using UnityEngine;
using UnityEditor;
public class TexturePostProcessor : AssetPostprocessor
{
void OnPostprocessTexture(Texture2D texture)
{
TextureImporter importer = assetImporter as TextureImporter;
...
Assume you have a custom file you want to create an importer for. It could be an .xls file or whatever. In this case we're going to use a JSON file because it's easy but we're going to pick a custom extension to make it easy to tell which files are ours?
Let's assume the format of the JSON file is
...
