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For any item without a specific page setup, it will use the default one. So make sure you always have a default page setup.
Here are the updated Missing File__input:Input-forms.dtd and File__input and Missing File: Input-forms.xml which which shows you some examples.
Please note that the 'hint' in this input-forms.xml is using i18n message instead of the help texts come with DSpace package. If you have multiple collections and would like to italicize the text or put a link in the hint, you will find this is very helpful. You don't have to maintain the same 'hint' for the fields in different places. Also it is possible to put the html tags around the text to format the hints.
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