[Zope] Problems in file upload

Good morning, anyone ever had problems with the standard template file_edit_form "Add new element" - "File"? With some files (pdf) I obtain "Document contains no data" with Firefox, while Explorer gives me a blank page. I discovered it does NOT depends on file size (I uploaded without any problems a 5 MB file and I can't upload one of 1.6 MB). I never saw this problem on other sites I created with Plone, but only in this server. Nello.