ANSWER: Lists files need to be saved as a .CSV (Comma delimited) file in Excel with UTF-8 encoding.
Each column should have a header that indicates what data is below it. See below for steps and examples.
1. Open your list in Microsoft Excel
2. Right click on the first row and select Insert to add a row into the top of your list
3. Type in header names for each column
4. Once finished, click File - Save As.
5. Select CSV (Comma delimited) under "Save as type" before saving the file
6. Click "OK" when given warning about "multiple sheets"
7. Click "Yes" when asked if you wish to keep workbook in current format
8. When exiting Excel, you will be asked if you wish to "Save changes" even though none were made. Click "Don't Save"