> For clean Markdown of any page, append .md to the page URL. > For a complete documentation index, see https://contentful.com/developers/docs/llms.txt. > For AI client integration (Claude Code, Cursor, etc.), connect to the MCP server at https://contentful.com/developers/docs/_mcp/server. # UI extensions - File structure > File structure of a UI Extension | Using an HTML file to hold the code and JSON file for supported field types > **Info** > > **Use the [App Framework](/extensibility/app-framework) for any new projects.** We recommend creating apps instead of UI extensions. [Read the app FAQ to learn why](https://www.contentful.com/help/contentful-app-framework-faq/). The minimal file structure of extensions consists of two files: * An HTML file (e.g. `index.html`) which holds the code of the extension * A JSON file (e.g. `extension.json`) which describes the extension's properties such as `name` or the supported field types Let's take a closer look at these files to understand what they do. ## Source code: index.html The `index.html` file is the building block for an extension and it is embedded into the Contentful entry editor as an `