show file extensions in Unity hierarchy view, support one/two column and thumbnail

FileExtensions.cs

using System;
using UnityEngine;
using UnityEditor;
using System.IO;
using System.Reflection;
using Object = UnityEngine.Object;

[InitializeOnLoad]
public static class FileExtensions
{
    public static Color color;

    private static string prevGuid;
    private static GUIStyle labelStyle;
    private static bool isPro;
    private static EditorWindow projectWindow;
    private static float size = 2.0f;
    private static float halfSize = 1.0f;
    private static int _columnsCount = 0;
    private static GUIStyle _style;
    private static string _selectedGuid;
    private static GUIContent _content;

    const int ONE_COLUMN = 0;
    const int TWO_COLUMNS = 1;

    static FileExtensions()
    {
        if (EditorGUIUtility.isProSkin)
        {
            color = Color.grey;
        }
        else
        {
            color = Color.white;
        }

        EditorApplication.projectWindowItemOnGUI += ListItemOnGUI;
        Selection.selectionChanged += () =>
        {
            if (Selection.activeObject != null)
                AssetDatabase.TryGetGUIDAndLocalFileIdentifier(Selection.activeObject, out _selectedGuid, out long id);
        };
    }

    private static void ListItemOnGUI(string guid, Rect rect)
    {
        if (Event.current.type != EventType.Repaint) return;
        if (prevGuid == guid) return;
        prevGuid = guid;

        labelStyle ??= new GUIStyle(EditorStyles.boldLabel)
        {
            normal =
            {
                textColor = color
            }
        };

        var assetPath = AssetDatabase.GUIDToAssetPath(guid);
        var obj = AssetDatabase.LoadAssetAtPath<UnityEngine.Object>(assetPath);

        if (obj != null && AssetDatabase.IsMainAsset(obj) &&
            !(obj is DefaultAsset && !AssetDatabase.IsForeignAsset(obj)))
        {
            string extension = Path.GetExtension(assetPath);

            if (IsThumbnailsView)
            {
                labelStyle.normal.textColor = Color.black;

                rect.x -= halfSize;
                EditorGUI.LabelField(rect, extension, labelStyle);

                rect.x += size;
                EditorGUI.LabelField(rect, extension, labelStyle);

                rect.x -= halfSize;
                rect.y -= halfSize;
                EditorGUI.LabelField(rect, extension, labelStyle);

                rect.y += size;
                EditorGUI.LabelField(rect, extension, labelStyle);

                rect.y -= halfSize;
                labelStyle.normal.textColor = Color.white;
                EditorGUI.LabelField(rect, extension, labelStyle);
                labelStyle.normal.textColor = color;
            }
            else
            {
                var fileName = Path.GetFileNameWithoutExtension(assetPath);
                _style ??= new GUIStyle(EditorStyles.label);
                var selected = _selectedGuid?.Length > 7 && guid?.Length > 7 &&
                               string.Compare(guid, 0, _selectedGuid, 0, 6) == 0;

                _style.normal.textColor = selected ? new Color32(255, 255, 255, 255) : new Color32(220, 220, 220, 220);

                _content ??= new GUIContent();
                _content.text = extension;
                var extSize = _style.CalcSize(_content);
                _content.text = fileName;
                var nameSize = _style.CalcSize(_content);
                rect.x += nameSize.x + (IsTwoColumns ? 18 : 15);
                rect.width = nameSize.x + 1 + extSize.x;

                var offsetRect = new Rect(rect.position, rect.size);
                EditorGUI.LabelField(offsetRect, extension, _style);
            }
        }
    }

    private static bool IsThumbnailsView
    {
        get
        {
            projectWindow = GetProjectWindow();
            PropertyInfo gridSize = projectWindow.GetType()
                .GetProperty("listAreaGridSize", BindingFlags.Instance | BindingFlags.Public);
            _columnsCount = (int)projectWindow.GetType()
                .GetField("m_ViewMode", BindingFlags.Instance | BindingFlags.NonPublic).GetValue(projectWindow);
            return _columnsCount == TWO_COLUMNS && (float)gridSize.GetValue(projectWindow, null) > 16f;
        }
    }
    
    private static bool IsTwoColumns
    {
        get
        {
            projectWindow = GetProjectWindow();
            PropertyInfo gridSize = projectWindow.GetType()
                .GetProperty("listAreaGridSize", BindingFlags.Instance | BindingFlags.Public);
            _columnsCount = (int)projectWindow.GetType()
                .GetField("m_ViewMode", BindingFlags.Instance | BindingFlags.NonPublic).GetValue(projectWindow);
            return _columnsCount == TWO_COLUMNS;
        }
    }

    private static EditorWindow GetProjectWindow()
    {
        if (EditorWindow.focusedWindow != null && EditorWindow.focusedWindow.titleContent.text == "Project")
        {
            return EditorWindow.focusedWindow;
        }

        return GetExistingWindowByName("Project");
    }

    private static EditorWindow GetExistingWindowByName(string name)
    {
        EditorWindow[] windows = Resources.FindObjectsOfTypeAll<EditorWindow>();
        foreach (EditorWindow item in windows)
        {
            if (item.titleContent.text == name)
            {
                return item;
            }
        }

        return default(EditorWindow);
    }
}