Upgrading from Mojave to a newer version of the Mac Operating System resolves this issue.

See https://www.apple.com/uk/macos/how-to-upgrade/

It is confirmed fixed in the Big Sur version of MacOS

However if it is not possible to do this, then the workaround should still be applied.

Attaching a file to Safari file picker

You can by-pass the file picker dialog by choosing a file via the Finder and dropping directly on to the "Choose file..." button (it should highlight if you have targetted it correctly).

This will do the same thing as if you had navigated through the file picker and selected the file.