file = request.files['file']
img = file.read()
img = cv2.imdecode(np.frombuffer(img, np.uint8), cv2.IMREAD_COLOR)

请注意如果这里用

cv2.imread()
则会报错

SystemError: <built-in function imread> returned NULL without setting an error

参考https://www.jianshu.com/p/cafbeec49674