how is it possible to write a filter for directories?

Hi,

I'd like to add extraordinary information for directories. In every one
of them a file named content.xml contains these information. 

After reading the documentation, I guess a filter is supposed to do this
job. Is that right? But what is the mime-type for directories? And what
is within the data blob for them?

Or is it better to rename my content.xml to something like content.umx and
implement a filter for *.umx that add certain flags to the directory
indexable where the file is located?

Thanks for Help.

Bye
 
    Max