Can not re-use the same field multiple times in the same content type (i.e with Flexifield)

Issue Summary

This is most easily reproduced installing flexifield, creating a content type containing only a file (say "file") and a type containing a flexifield referencing the first type (say "fileset"). Whenever saving a form, the subsequent files or images are overwritten by the first one.

This is because the rendered html field (<input type="file" ...>) does not respect the #array_parents in its naming. The supplied patch fixes this.