[IMP] fix mime type in case of excel export
[IMP] fix mime type in case of excel export

akretion/openerp-web,havt/openerp-web,akretion/openerp-web,havt/openerp-web,akretion/openerp-web,havt/openerp-web,havt/openerp-web,havt/openerp-web