Error messaging: Content of file must be binary

In an enterprise application's bit editor function, we want our users to be able to import configuration files.

The files are supposed to only contain binary content - "0" and "1"s - but I initially had the error message read "Content of file must be binary", but the pushback was that the users may mistake that to mean that it must be a .bin file.

Could I have some assistance in better messaging on this?

Thank you!