TypeError: Object of type 'bytes' is not JSON serializable

[liudongdong1]

when i try this code,and i meet an error about encoding,and i dont know how to do with is?
``@itchat.msg_register([PICTURE])
def download_files(msg):
print('recieved')
parm_translation_origin_language = 'zh' # will be overwriten by TEXT_DETECTION
msg.download(msg.fileName)
print('\nDownloaded image file name is: %s' % msg['FileName'])
with open(msg['FileName'], "r") as image_file:
image_content = image_file.read()
return base64.b64encode(image_content)

and the error is:
Traceback (most recent call last):
File "D:\Anaconda3\lib\site-packages\itchat\components\register.py", line 60, in configured_reply
r = replyFn(msg)
File "", line 10, in download_files
image_content = image_file.read()
UnicodeDecodeError: 'gbk' codec can't decode byte 0xff in position 0: illegal multibyte sequence

i make some change,but it didn't work,
with open(msg['FileName'], "rb") as image_file:
image_content = image_file.read()
return base64.b64encode(image_content)
thank you for your time!