-- HTML and CSS part.

• The HTML(index.html) contains a form for changing the file extension of a selected file. It also includes CSS styling for a glass frame effect and a background image, as well as a JavaScript file for handling the form submission.

-- Javascript part(script.js).

• The function changes the extension of an uploaded file and creates a download link for the new file.
• @returns Nothing is being returned. The function is modifying the DOM by adding a download button for the new file.