uploaded files are deleted automatically ?

Hi,

I am trying to modify og_fiies module to make it work with D6, however, I am encountering a strange problem:

each time, when I upload a file with og_files module,

I insert some record into {og_files} table and {files} table.

It seems to me, after a few days, the uploaded file will be automatically deleted from my file system, and the record of that file in {files} table is also deleted.

I checked the code in og_files module, and did not find the clue for this.

Anybody has ideas for this ?

Thanks

Hang