Check if file already exists is not working

Problem/Motivation

The check if file with given name already exists on the disc is not working.

Steps to reproduce

Try to save a file with a name of already existing file.

Proposed resolution

Add a check (was removed in 1.0.0, I guess by by accident)

Remaining tasks

Commit changes, add patch release.

User interface changes

None.

API changes

None.

Data model changes

None.