in the form, the hidden element is named "hiddenField";
<input name="hiddenField" type="hidden" id="hiddenField" value="<?php echo $row_rs_fichier['IDfichier']; ?>" />
in the recordset, you have set the filter to use the IDfichier column and a form element named "IDfichier", which does not exist.
you will need to change the name of the form element:
<input name="IDfichier" type="hidden" id="IDfichier" value="<?php echo $row_rs_fichier['IDfichier']; ?>" />
in the Download behavior, you have set the File name to use the IDfichier column from the recordset Is that column the ID column or the file name column?