Data file does not match the specified file type (&1 / &2)

What causes this issue?

You want to import a data file. Before the file is actually imported,
the system starts reading the file to validate the file type. This file
type must match the file type specified by the user. However, the data
file does not match the specified file type.

System Response

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

How to fix this error?

Verify that the chosen file type matches the data file.
Verify that the data file matches the file type specification.

Procedure for System Administrators

Check the settings in Customizing. Changes to customer-specific
coding may be necessary.

Error message extract from SAP system. Copyright SAP SE.