Repository: godotengine/godot
File: editor/docks/filesystem_dock.cpp (C++)

Code before:
			Ref<Texture2D> thumbnail;

			if (file_list_display_mode == FILE_LIST_DISPLAY_LIST) {
				thumbnail = p_small_preview;
			} else {
				thumbnail = p_preview;
			}

			if (thumbnail.is_valid()) {
				files->set_item_icon(p_index, _apply_thumbnail_filter(thumbnail, p_path));
			}
		}
	}
}

void FileSystemDock::_tree_thumbnail_done(const String &p_path, const Ref<Texture2D> &p_preview, const Ref<Texture2D> &p_small_preview, int p_update_id, ObjectID p_item) {
	TreeItem *item = ObjectDB::get_instance<TreeItem>(p_item);
	if (item && tree_update_id == p_update_id && p_small_preview.is_valid()) {
		item->set_icon(0, _apply_thumbnail_filter(p_small_preview, p_path));
	}
}

Ref<CanvasTexture> FileSystemDock::_apply_thumbnail_filter(const Ref<Texture2D> p_thumbnail, const String &p_file_path) const {
	Ref<CanvasTexture> thumbnail_wrapped;
	thumbnail_wrapped.instantiate();
	thumbnail_wrapped->set_diffuse_texture(p_thumbnail);

	if (!p_file_path.is_empty()) {
		int index;
		EditorFileSystemDirectory *dir = EditorFileSystem::get_singleton()->find_file(p_file_path, &index);

		if (dir) {
			if (dir->get_file_import_is_valid(index)) {
				const String &file_type = dir->get_file_type(index);

				if (file_type == SNAME("CompressedTexture2D") || file_type == SNAME("Image")) {
					const String extension = p_file_path.get_extension();

					if (extension != "svg" && extension != "svgz") {
						thumbnail_wrapped->set_texture_filter(CanvasItem::TextureFilter::TEXTURE_FILTER_NEAREST_WITH_MIPMAPS);
					}
				}
			}
		}
	}

	return thumbnail_wrapped;
}

void FileSystemDock::_toggle_file_display() {
	_set_file_display(file_list_display_mode != FILE_LIST_DISPLAY_LIST);

Review comment (suggestion):
You could make the method return Texture2D and only make CanvasTexture when you have to apply the filter.

Code after:
			Ref<Texture2D> thumbnail;

			if (file_list_display_mode == FILE_LIST_DISPLAY_LIST) {
				thumbnail = p_small_preview;
			} else {
				thumbnail = p_preview;
			}

			if (thumbnail.is_valid()) {
				files->set_item_icon(p_index, _apply_thumbnail_filter(thumbnail, p_path));
			}
		}
	}
}

void FileSystemDock::_tree_thumbnail_done(const String &p_path, const Ref<Texture2D> &p_preview, const Ref<Texture2D> &p_small_preview, int p_update_id, ObjectID p_item) {
	TreeItem *item = ObjectDB::get_instance<TreeItem>(p_item);
	if (item && tree_update_id == p_update_id && p_small_preview.is_valid()) {
		item->set_icon(0, _apply_thumbnail_filter(p_small_preview, p_path));
	}
}

Ref<Texture2D> FileSystemDock::_apply_thumbnail_filter(const Ref<Texture2D> &p_thumbnail, const String &p_file_path) const {
	if (!p_file_path.is_empty()) {
		int index;
		EditorFileSystemDirectory *dir = EditorFileSystem::get_singleton()->find_file(p_file_path, &index);

		if (dir) {
			if (dir->get_file_import_is_valid(index)) {
				const StringName &file_type = dir->get_file_type(index);

				if (file_type == SNAME("CompressedTexture2D") || file_type == SNAME("Image")) {
					const String extension = p_file_path.get_extension();

					if (extension != "svg" && extension != "svgz") {
						Ref<CanvasTexture> thumbnail_wrapped;
						thumbnail_wrapped.instantiate();
						thumbnail_wrapped->set_diffuse_texture(p_thumbnail);
						thumbnail_wrapped->set_texture_filter(CanvasItem::TextureFilter::TEXTURE_FILTER_NEAREST_WITH_MIPMAPS);
						return thumbnail_wrapped;
					}
				}
			}
		}
	}

	return p_thumbnail;
}

void FileSystemDock::_toggle_file_display() {
	_set_file_display(file_list_display_mode != FILE_LIST_DISPLAY_LIST);