An example of a really simple asset importer that just reads in text files with a custom .omni extension

OmniAsset.cs

using UnityEngine;

#if UNITY_EDITOR
using UnityEditor.AssetImporters;
using System.IO;

[ScriptedImporter(1, "omni")]
public class OmniAssetImporter : ScriptedImporter
{
    public override void OnImportAsset(AssetImportContext ctx)
    {
        var asset = ScriptableObject.CreateInstance<OmniAsset>();
        asset.text = File.ReadAllText(ctx.assetPath);

        ctx.AddObjectToAsset("main obj", asset);
        ctx.SetMainObject(asset);
    }
}
#endif

public class OmniAsset : ScriptableObject
{
    public string text = "";
}