Editor\ShowFileExtensions.cs

using System.Collections.Generic;
using System.IO;
using UnityEngine;
using UnityEditor;

[InitializeOnLoad]
public static class ShowFileExtensions
{
    static ShowFileExtensions()
    {
        EditorApplication.projectWindowItemOnGUI += Draw;
    }

    static HashSet<string> _done = new HashSet<string>();
    static float _lastY;
    
    static void Draw( string pGuid, Rect pRect )
    {
        // if we've restarted drawing, clear list of paths we've done
        if( pRect.y < _lastY )
            _done.Clear();

        var path = AssetDatabase.GUIDToAssetPath( pGuid );

        // don't show extension if we've already shown it for this path
        // which prevents us showing extensions on sub-assets
        if( !_done.Add( path ) )
            return;
        
        _lastY = pRect.y;

        var filename = Path.GetFileNameWithoutExtension( path );
        var extension = Path.GetExtension( path );

        if( pRect.height > 20 )
        {
            // icon mode
            
            var size = EditorStyles.miniLabel.CalcSize( new GUIContent( extension ) );
            
            pRect.x += pRect.width / 2 - size.x / 2;
            pRect.width = size.x;
            pRect.y += pRect.height - 2;
            pRect.height = size.y;
            
            EditorGUI.LabelField( pRect, extension, EditorStyles.miniLabel );
        }
        else
        {
            // single column mode
            
            var size = EditorStyles.label.CalcSize( new GUIContent( filename ) );
            pRect.x += size.x + 11;
            pRect.width -= size.x + 11;
            pRect.y += 1;
            
            EditorGUI.LabelField( pRect, extension );
        }
    }
}