Rails API ASCII Conversion Error with Base64 decoded images

I have a Python script, that sends an image over a POST-Request to a Rails API. The images is Base64 encoded and then UTF-8 encoded. Otherwise the Request errors with the following error:

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

The Python script looks like this:

with open('C:\\Users\\maforlkzus\\Desktop\\test.jpg', 'rb') as f:
    encoded_image = base64.b64encode(f.read())
    image = encoded_image.decode('utf-8')
payload = {
    'name': 'testimage',
    'image': image,
}

r = requests.post(url, data=json.dumps(payload), headers={'Content-type': 'application/json'})

In my Rails app, I want to create a Temporary File which holds the image. Therefore I have to base64-decode the image, but that does not work because of the UTF-8 encoding. My Rails controller looks like this:

1 def decode_file
2   temp_file = Tempfile.new('test')
3   testfile = self.image.force_encoding('utf-8')
4   temp_file.write(Base64.decode64(testfile))
5   self.file = temp_file
6 end

>>> Encoding::UndefinedConversionError ("\xFF" from ASCII-8BIT to UTF-8): line 4 in decode_file

If I try to decode it like this, I get the same error:

def decode_file
    temp_file = Tempfile.new('test')
    temp_file.write(Base64.decode64(self.image))
    self.file = temp_file
end

How can I fix this? Do I have to encode the image differently before sending or is the problem in the API code?