Amazon S3 Chrome vs. Firefox Bucket POST enclosure-type multipart/form-data

I have a form that makes a POST request directly to an Amazon S3 bucket with a file attachment. In Firefox it works fine. In Chrome, I get a 412 error

<?xml version="1.0" encoding="UTF-8"?>
<Error>
    <Code>PreconditionFailed</Code>
    <Message>At least one of the pre-conditions you specified did not hold</Message>
    <Condition>Bucket POST must be of the enclosure-type multipart/form-data</Condition>
    <RequestId>randomcharacters123456</RequestId>
    <HostId>morerandomcharacters123456</HostId>

</Error>

Is there supposed to be a difference in the way these browsers handle and submit forms? The form has the attribute enctype=multipart/form-data set fine. The only real difference I saw in the POST requests was the request payload boundary.

Chrome shows something like ------WebKitFormBoundary27B45bsnJm6YBk66
Firefox shows something like -----------------------------185573058924207

But I assume these are trivial differences.

Do I have to cater enctypes to particular browsers? Can I force Chrome to handle the enctype properly?