[horde] strange vfs files in /tmp

[AJ]

Hello,
   Even after setting up a file based vfs backend pointing to a directory 
other than /tmp, I am getting strange vfsxxxxxx files in the /tmp directory.
They all start with vfs and have a number of random characters as the last 
part of the filename.  Does anyone know what these are?
Note, IMP attachments are being stored in the proper directory I specified.

Thanks.
AJ