Here is the deal:
A component (SOBI2) was installed in our Joomla site. We also installed a plugin for this component to make it a download manager.
To upload a file in frontend, user must start the process to add new entry, and then click in link that opens an ajax popup and there choose and upload the file, and then close this popup and send the "add entry" form.
The Problem: When someone try to upload a file via frontend, the file is not sent to the database and if later someone tries to send another file, when he opens the upload popup, the file that the other user tried to send is there.