Upload file trouble while updating record.

Hi,
I apologies for my bad english speaking.
I saw several times this problem on the forums but I can't fix it in my pages. I've tryed to follow this tutorial : dfp_da_insert_update_ht.pdf

When I replace the default input type to "hidden" instead of "text", the recorded name of my image stay good. That's a good point. BUT…
The problem is that when I try to update a record in my database with a new file field called "imageNEW" for example, the file doesn't upload, and it doesn't record the filename in the database.
If i just change the default input type to "file" instead as "text" it works only if I send another file. If I leave the field blank, the existant filename is erased from the database and the image cannot be displayed.

I don't understand what I have to do.
I've buy this extensions to help me developping websites by my own, but I can't…
And I need help :)