All file uploads must be in JAR (Java Archive) format. In cases where one or more auxiliary files are intended to work with the main JAR file, these can be uploaded as additional (child) files from the main file's information page.
To upload an Additional File, go to the Files page on your project, then go to the page for a specific file. Near the bottom of that page, under the Changelog section you will see this:
Click the Upload Additional File button, then enter the appropriate information and upload the file as you would normally.