Incorrect field name in error message during image upload on "Images" tab for category items

On image upload form, when non-image is uploaded (e.g. php file instead of jpg file), then error message "undefined - Invalid File Format" is displayed. "undefined" is shown instead of correct field name.

This happens because we have custom block for image upload on that template, where correct phrase is not defined for given upload field.

Mentioned above has effect in all places, where "Images" tab is present on item edit page in administrative console (categories, category items).