File store module - Overwriting Filename....Help needed

hi,

I have a Drupal v4.2. In the filestore module, i want to assign the filename which is gooin to the database as a custom value...
That is the filename obtained from the file field "upload_file"
is assigned to the filenam to be stored in the database...

i want a filename i specify, say

filename="myfilename" .......where should i specify the custom filename so that the filename in the database is written as "myfilename" instead of the filename obtained from the "upload_file" file box

TIA

harish