Think links all the executables to KioFileWidgets. This could result in a slower build.
Please add a dedicated ecm_add_test() below for this new test.

Maybe this button could have the "option" icon? (the one used also in the Settings menu)

i18nc("@action:button", "Add")

Is this really necessary?

optionsWidget is a child of optionsDialog, no need to delete it manually.

checked is not used, we can drop it (the connect() will work fine).

Q_NULLPTR instead of 0

Can't you add this widget in createdialog.ui? You can call setMimeType() instead of passing the mimetype to the constructor.

The name of the actions and the title of the dialog are not consistent:

• Action in the menu => Add File...
• Action in the toolbar => Add File
• Title of the dialog => Add Files

Not sure what should we do, though...

No

If the result of the action (through a dialog in this case) allows to add multiple files, then it should be Add Files, yes - but no ellipsis in the dialog title, only in the menu.

Ok, so the menu item and toolbar item should have ellipsis, but not the dialog title?

Yes:
"Indicate a function that needs additional information (including a confirmation) by adding an ellipsis at the end of the label (e.g. Save as…)."
(this is not only for the KDE world afaik)
https://techbase.kde.org/Projects/Usability/HIG/Menu_Bar