'.jpg' or '.png' sequence to a list

seqToList.py

def seqToArray(path):
    data = []
    for i in range(1491):
        if os.listdir(path)[0][-3:] == 'jpg':
            data.append(array(Image.open(path + '{}'.format(i+1) + '.jpg')))
        elif os.listdir(path)[0][-3:] == 'png':
            data.append(array(Image.open(path + '{}'.format(i+1) + '.png')))
        else :
            print('there is no "jpg" jr "png" inside')
    return data

#images to list
x = seqToArray('hack_online_data/train/')
#mask to list
y = seqToArray('hack_online_data/train_mask/')