There are a number of ways to drag and drop files for CodePen to use!

1) The Pen Editor

You can drag files from your computer right onto the editors and the contents of the file will go there.

Little bonus: the editor will show you a cursor of where the contents of that file will go.

3) The Project Editor

The Project Editor sidebar accepts files and folders.

3) Assets

From the Asset Manager or the popup Assets dialog in the Pen or Project Editor, you can drag and drop files in to upload.