Attachments created by import from xar file are not stored in file system

Our Xwiki (13.10.4) is configured with “store attachments in file systems”.
But if a XAR-file containing document attachments is imported, the attachments are stored in the data base.

Especially macros containing something like
doc.getAttachment_content().storageFile.getAbsolutePath()
will not work (and macros running in the operating system using the path too)

The attachments have to uploaded additionally.

Is there any additional configuration we have missed ?