I know this is an old question but it's ranking high on search engines in regards to this issue.
properties in the FileList object cannot be deleted but at least on Firefox they can be changed. My workaround this issue was to add a property IsValid=true
to those files that passed check and IsValid=false
to those that didn't.
then I just loop through the list to make sure that only the properties with IsValid=true
are added to FormData.