title: Prepend BOM to plone.namedfile field
body: <p>I need to prepend the BOM to a NamedBlobFile field on my Dexterity type to make Windows users happy, but leave the file in blobstorage intact. </p>
  
  <p>What is the recommended way to hook into Zope's file streaming without changing the actual file on the filesystem?</p>