Thanks for joining the thread and letting us know your concern, Top Quality Motors.

The issue happened when the file association of the .iif file change to other applications other than MS Excel. You'll want to verify the type of file you are opening by right-clicking the file and choosing Properties. The file type is listed under Type of File.

You may also want to change the file association of the .iif file to open with MS Excel. This way, you'll be able to open the file.

To learn more about importing files in QuickBooks, visit these useful articles: 

• Tasks you can do with IIF files
• IIF Overview: import kit, sample files, and headers
• Export, import, and edit IIF files
• Export lists from the old company file into a new data file

Please know I'm always here if there's anything else I can do for you. The Community will always have your back. Have a great day!