Writing this in case it’s helpful for future readers:

I was struggling with a similar issue, where we used pre-signed URLs with the DigitalOcean spaces storage backend for multi-part file uploads.

Initially, the requests failed entirely due to lacking CORS policies in the DigitalOcean spaces responses. After some diggin, it turned out you can configure CORS policies in the DO spaces management dashboard.

Next issue we ran to is that the client (browser) didn’t have access to the ETag response header sent by DigitalOcean spaces, as it was lacking the Access-Control-Expose-Headers header allowing the access to the ETag header. To solve this, there doesn’t seem to be an official DigitalOcean supported way to do it, but luckily I found this thread which showed how you can do it directly with s3cmd:

With s3cmd, it’s possible to upload a CORS policy file directly to the bucket, which also works on DigitalOcean spaces. This overrides any existing CORS settings set on the bucket

Example CORS policy file:

<?xml version="1.0" encoding="UTF-8"?>
<CORSConfiguration xmlns="http://s3.amazonaws.com/doc/2006-03-01/">
    <CORSRule>
        <AllowedOrigin>*</AllowedOrigin>
        <AllowedMethod>POST</AllowedMethod>
        <AllowedMethod>PUT</AllowedMethod>
        <AllowedMethod>DELETE</AllowedMethod>
        <MaxAgeSeconds>3000</MaxAgeSeconds>
        <ExposeHeader>ETag</ExposeHeader>
        <AllowedHeader>*</AllowedHeader>
    </CORSRule>
</CORSConfiguration>

Note how we can define the ExposeHeader property here on top of the settings already manageable on the DigitalOcean dashboard. I do not know why has the option been omitted from the official UI.

Once you have the file and s3cmd configured, it’s simple enough to upload to the bucket:

s3cmd setcors FILE s3://BUCKET