When a user opens the "Edit File Info", doesn't make any changes, and hits submit the "Edit Summary" input field on the import preview page will become enabled even though it should remain disabled until the user actually makes a change.
This is appears to be due to the fact that when the Wikitext editor is called the data returned on a submit can contain different hidden characters (e.g. \n becomes \r\n). The check currently implemented in T160184 rightly indicates that the too strings are different, however, Mediawiki seems to ignore these changes, therefore, no revision is created and the now enabled edit summary is ignored.
A solution may be to implement the same check that Mediawiki uses to determine whether or not the submitted Wikitext is different and if so only then allow editing the summary. That way an edit summary can only be added if Mediawiki were to create a revision.