By "explorer" I assume you mean the filebrowser plugin. See the preferences to define hidden extensions.
aha, it's in **Tools** → **Plugin Manager** → **File Browser**: <img width="621" alt="图片" src="https://github.com/geany/geany/assets/1039073/af439e7e-d9fd-4b87-8b24-2b8f51b0bec9">