Thanks Axel,

if I set 

attachment="contract_fibra_30Mb.pdf"            

I have got this error 

            ValueError: "name 'contract_fibra_30Mb' is not defined" while evaluating

            u'contract_fibra_30Mb.pdf'

I think because I have to declare the name as a string, like this: 

attachment=" 'contract_fibra_30Mb.pdf' "     or

attachment=" 'contract_fibra_30Mb' + '.pdf' "     

Anyway, even in the second type of declaration, the output pdf still doesn't take the right name,ç

but still has this name: "contract_fibra_print.contract_f_30.pdf" 

Hi Andrea Alessandrini

The file attribute of the report definition tag it used to save it's value for the field report_rml of the model ir.actions.report.xml that have the following definition were you could read it's help

'report_rml': fields.char('Main Report File Path/controller', help="The path to the main report file/controller (depending on Report Type) or NULL if the content is in another data field"),

so it's not intended to be used with qweb reports since it will not be used at all.

To specify the report filename to be generated you need to use the attachment attribute that it will end in the attachment field of the same model that have the following definition:

'attachment': fields.char('Save as Attachment Prefix', help='This is the filename of the attachment used to store the printing result. Keep empty to not save the printed reports. You can use a python expression with the object and time variables.'),

Hope this helps