[Backport CMS7-8702 to 7.9] Image upload dialog: File select button not disabled after adding more than 25 files

The image upload dialog states 'You can select up to 25 files'
When selecting 30 images the OK button is not disabled, Seems to be no warning that the maximum number of files is reached. Only 25 images are imported without any feedback.

The dialog should block adding more images if 25 are already selected. If more are added the dialog must not close without giving feedback on which images are imported/rejected