how to reproduce

• go to the demo : https://demo.ametys.org/nightly-intranet-cms/intranet/index.html
• create a form
• insert a file field
• insert a phrase in the title wording a "l' " (see screenshot)
• make this field mandatory
• insert the form into a page
• go to preview
• click ok without adding an attachment
  the form turns in the void