A 72-year-old man with a long history of smoking presents with a 3-month history of progressive dysphagia, initially for solids and now for liquids. He has lost 5kg in weight. He also complains of retrosternal discomfort on swallowing. An endoscopy reveals a friable, ulcerated mass in the mid-oesophagus. Biopsies are taken. Which histological type of oesophageal cancer is most likely in this patient?