The download area needs to be "directed" as to which files can be added to the download area.
To add the file type that you would like to include:
1. Under 'Downloads' choose 'Configure DL Area'
2. In the form field 'File Types Allowed' add the file extension you would like to include
3. Be sure to separate each extension with a comma, and enter each extension in both lowercase and uppercase letters
4. Click 'Apply Changes to Download Area'