What are the editor resources by name for EditorGUIUtility.Load Method?

I don't think you need to know the location of the icons, nor their names if you want to get the preview for a certain object or type, nor if a script is JS or CS - you could just use AssetPreview.GetMiniThumbnail(UnityEngine.Object) and AssetPreview.GetMiniTypeThumbnail(System.Type)

If you still want the names, maybe you want to load the icons manually, I found this method in UnityEditorInternal.InternalEditorUtility, gives you all the icon names you need!

 public static Texture2D GetIconForFile(string fileName)
 {
     int num = fileName.LastIndexOf('.');
     string text = (num != -1) ? fileName.Substring(num + 1).ToLower() : string.Empty;
     string text2 = text;
     switch (text2)
     {
     case "boo":
         return EditorGUIUtility.FindTexture("boo Script Icon");
     case "cginc":
         return EditorGUIUtility.FindTexture("CGProgram Icon");
     case "cs":
         return EditorGUIUtility.FindTexture("cs Script Icon");
     case "guiskin":
         return EditorGUIUtility.FindTexture("GUISkin Icon");
     case "js":
         return EditorGUIUtility.FindTexture("Js Script Icon");
     case "mat":
         return EditorGUIUtility.FindTexture("Material Icon");
     case "prefab":
         return EditorGUIUtility.FindTexture("PrefabNormal Icon");
     case "shader":
         return EditorGUIUtility.FindTexture("Shader Icon");
     case "txt":
         return EditorGUIUtility.FindTexture("TextAsset Icon");
     case "unity":
         return EditorGUIUtility.FindTexture("SceneAsset Icon");
     case "asset":
     case "prefs":
         return EditorGUIUtility.FindTexture("GameManager Icon");
     case "anim":
         return EditorGUIUtility.FindTexture("Animation Icon");
     case "meta":
         return EditorGUIUtility.FindTexture("MetaFile Icon");
     case "ttf":
     case "otf":
     case "fon":
     case "fnt":
         return EditorGUIUtility.FindTexture("Font Icon");
     case "aac":
     case "aif":
     case "aiff":
     case "au":
     case "mid":
     case "midi":
     case "mp3":
     case "mpa":
     case "ra":
     case "ram":
     case "wma":
     case "wav":
     case "wave":
     case "ogg":
         return EditorGUIUtility.FindTexture("AudioClip Icon");
     case "ai":
     case "apng":
     case "png":
     case "bmp":
     case "cdr":
     case "dib":
     case "eps":
     case "exif":
     case "gif":
     case "ico":
     case "icon":
     case "j":
     case "j2c":
     case "j2k":
     case "jas":
     case "jiff":
     case "jng":
     case "jp2":
     case "jpc":
     case "jpe":
     case "jpeg":
     case "jpf":
     case "jpg":
     case "jpw":
     case "jpx":
     case "jtf":
     case "mac":
     case "omf":
     case "qif":
     case "qti":
     case "qtif":
     case "tex":
     case "tfw":
     case "tga":
     case "tif":
     case "tiff":
     case "wmf":
     case "psd":
     case "exr":
         return EditorGUIUtility.FindTexture("Texture Icon");
     case "3df":
     case "3dm":
     case "3dmf":
     case "3ds":
     case "3dv":
     case "3dx":
     case "blend":
     case "c4d":
     case "lwo":
     case "lws":
     case "ma":
     case "max":
     case "mb":
     case "mesh":
     case "obj":
     case "vrl":
     case "wrl":
     case "wrz":
     case "fbx":
         return EditorGUIUtility.FindTexture("Mesh Icon");
     case "asf":
     case "asx":
     case "avi":
     case "dat":
     case "divx":
     case "dvx":
     case "mlv":
     case "m2l":
     case "m2t":
     case "m2ts":
     case "m2v":
     case "m4e":
     case "m4v":
     case "mjp":
     case "mov":
     case "movie":
     case "mp21":
     case "mp4":
     case "mpe":
     case "mpeg":
     case "mpg":
     case "mpv2":
     case "ogm":
     case "qt":
     case "rm":
     case "rmvb":
     case "wmw":
     case "xvid":
         return EditorGUIUtility.FindTexture("MovieTexture Icon");
     case "colors":
     case "gradients":
     case "curves":
     case "curvesnormalized":
     case "particlecurves":
     case "particlecurvessigned":
     case "particledoublecurves":
     case "particledoublecurvessigned":
         return EditorGUIUtility.FindTexture("ScriptableObject Icon");
     }
     return EditorGUIUtility.FindTexture("DefaultAsset Icon");
 }

EDIT:

Not icons, but also relevant - some GUI internal styles including the toolbarSearchField (the search field that's used in the hierarchy and project views)

 // UnityEditor.EditorStyles
 private void InitSharedStyles()
 {
     this.m_ColorPickerBox = this.GetStyle("ColorPickerBox");
     this.m_InspectorBig = this.GetStyle("In BigTitle");
     this.m_MiniLabel = this.GetStyle("miniLabel");
     this.m_LargeLabel = this.GetStyle("LargeLabel");
     this.m_BoldLabel = this.GetStyle("BoldLabel");
     this.m_MiniBoldLabel = this.GetStyle("MiniBoldLabel");
     this.m_WordWrappedLabel = this.GetStyle("WordWrappedLabel");
     this.m_WordWrappedMiniLabel = this.GetStyle("WordWrappedMiniLabel");
     this.m_WhiteLabel = this.GetStyle("WhiteLabel");
     this.m_WhiteMiniLabel = this.GetStyle("WhiteMiniLabel");
     this.m_WhiteLargeLabel = this.GetStyle("WhiteLargeLabel");
     this.m_WhiteBoldLabel = this.GetStyle("WhiteBoldLabel");
     this.m_MiniTextField = this.GetStyle("MiniTextField");
     this.m_RadioButton = this.GetStyle("Radio");
     this.m_MiniButton = this.GetStyle("miniButton");
     this.m_MiniButtonLeft = this.GetStyle("miniButtonLeft");
     this.m_MiniButtonMid = this.GetStyle("miniButtonMid");
     this.m_MiniButtonRight = this.GetStyle("miniButtonRight");
     this.m_Toolbar = this.GetStyle("toolbar");
     this.m_ToolbarButton = this.GetStyle("toolbarbutton");
     this.m_ToolbarPopup = this.GetStyle("toolbarPopup");
     this.m_ToolbarDropDown = this.GetStyle("toolbarDropDown");
     this.m_ToolbarTextField = this.GetStyle("toolbarTextField");
     this.m_ToolbarSearchField = this.GetStyle("ToolbarSeachTextField");
     this.m_ToolbarSearchFieldPopup = this.GetStyle("ToolbarSeachTextFieldPopup");
     this.m_ToolbarSearchFieldCancelButton = this.GetStyle("ToolbarSeachCancelButton");
     this.m_ToolbarSearchFieldCancelButtonEmpty = this.GetStyle("ToolbarSeachCancelButtonEmpty");
     this.m_SearchField = this.GetStyle("SearchTextField");
     this.m_SearchFieldCancelButton = this.GetStyle("SearchCancelButton");
     this.m_SearchFieldCancelButtonEmpty = this.GetStyle("SearchCancelButtonEmpty");
     this.m_HelpBox = this.GetStyle("HelpBox");
     this.m_AssetLabel = this.GetStyle("AssetLabel");
     this.m_AssetLabelPartial = this.GetStyle("AssetLabel Partial");
     this.m_AssetLabelIcon = this.GetStyle("AssetLabel Icon");
     this.m_SelectionRect = this.GetStyle("selectionRect");
     this.m_MinMaxHorizontalSliderThumb = this.GetStyle("MinMaxHorizontalSliderThumb");
     this.m_BoldFont = this.GetStyle("BoldLabel").font;
     this.m_StandardFont = this.GetStyle("Label").font;
     this.m_MiniFont = this.GetStyle("MiniLabel").font;
     this.m_MiniBoldFont = this.GetStyle("MiniBoldLabel").font;
     this.m_ProgressBarBack = this.GetStyle("ProgressBarBack");
     this.m_ProgressBarBar = this.GetStyle("ProgressBarBar");
     this.m_ProgressBarText = this.GetStyle("ProgressBarText");
     this.m_FoldoutPreDrop = this.GetStyle("FoldoutPreDrop");
     this.m_InspectorTitlebar = this.GetStyle("IN Title");
     this.m_InspectorTitlebarText = this.GetStyle("IN TitleText");
     this.m_ToggleGroup = this.GetStyle("BoldToggle");
     this.m_NotificationText = this.GetStyle("NotificationText");
     this.m_NotificationBackground = this.GetStyle("NotificationBackground");
     this.m_Popup = (this.m_LayerMaskField = this.GetStyle("MiniPopup"));
     this.m_TextField = (this.m_NumberField = this.GetStyle("textField"));
     this.m_Label = this.GetStyle("ControlLabel");
     this.m_ObjectField = this.GetStyle("ObjectField");
     this.m_ObjectFieldThumb = this.GetStyle("ObjectFieldThumb");
     this.m_Toggle = this.GetStyle("Toggle");
     this.m_ToggleMixed = this.GetStyle("ToggleMixed");
     this.m_ColorField = this.GetStyle("ColorField");
     this.m_Foldout = this.GetStyle("Foldout");
     this.m_FoldoutSelected = GUIStyle.none;
     this.m_TextArea = new GUIStyle(this.m_TextField);
     this.m_TextArea.wordWrap = true;
     this.m_InspectorDefaultMargins = new GUIStyle();
     this.m_InspectorDefaultMargins.padding = new RectOffset(14, 4, 0, 0);
     this.m_InspectorFullWidthMargins = new GUIStyle();
     this.m_InspectorFullWidthMargins.padding = new RectOffset(5, 4, 0, 0);
 }