A 52-year-old man with a 20-year history of poorly controlled type 2 diabetes presents with progressive dyspnea and bilateral lower-extremity edema. Echocardiography shows a preserved ejection fraction of 60% with grade III diastolic dysfunction and biatrial enlargement. Endomyocardial biopsy stained with Congo red shows apple-green birefringence under polarized light. Which of the following is the most likely cause of his cardiac findings?