"This workbook contains links to one or more external sources that could be unsafe" when uploading an Excel file

Issue
I am uploading Excel spreadsheets to FYI and getting the following message:

"This workbook contains links to one or more external sources that could be unsafe"

Cause
This message is a Security Warning that the automatic update of links has been disabled in Excel. A spreadsheet may have a link to another workbook however it can no longer access these and cannot update the links automatically. The existing links are therefore considered unsafe.

1. Open the Excel spreadsheet.
2. Click File.
3. Click Info.
4. Click Edit Links to Files.
5. In the Edit Links pop-up, if relevant select the link and click Break Link and confirm.
6. Save the spreadsheet.

Alternatively, to suppress the message so this applies to all Excel spreadsheets and to automatically update the links when you open a workbook, follow these steps:  

1. In Excel, click File.
2. Click Options.
3. Click Advanced.
4. Under General, click to disable the checkbox Ask to update automatic links.
5. Click OK.