First of all, these two articles contain everything that was necessary to guide me to the answer:

• https://upcloud.com/docs/guides/connecting-to-an-object-storage-instance-s3cmd/
• https://filezillapro.com/how-to-connect-to-digitalocean-spaces/

Everything written here applies to the new UpCloud Object Storage, not the one labeled EOL.

Before connecting FileZilla to your UpCloud S3 instance, that instance of course must be created in the UpCloud Object Storage Management page and have a user attached to it. That's the easy part, UpCloud provides extensive docs to this end.

On creating an Object Storage instance, a subdomain is generated (e.g. a1234) on upcloudobjects.com.
On creating the user, an Access Key ID and a Secret Access Key are generated.

Below are the UpCloud Object Storage parameters used in the following example. Replace them with the actual parameters from your instance.

• S3 endpoint (e.g. https://a1234.upcloudobjects.com)
• Region (e.g. europe-1)
• Access Key ID (20 characters)
• Secret Access Key ID (40 charactres)

Note that a1234 in all examples must be replaced with the actual subdomain name of your instance.

Preparing Filezilla

1. Adding the UpCloud S3 Provider to FileZilla

FileZilla must know about our UpCloud provider. Open Edit > Settings, select Transfers > S3:Providers.

1. Add a new provider and give it a name ('UpCloud a1234' or whatever name is apt);
2. add the region (in this example: europe-1, description EUROPE-1, endpoint a1234.upcloudobjects.com, without https://);
3. for 'Catch all' provide the value .a1234.upcloudobjects.com (note the dot at the front);
4. for 'Format' provide the value %(bucket)s.a1234.upcloudobjects.com;
5. save (click OK).

2. Setting FileZilla S3 Transfer options

Before adding any objects to the store, a few options must be set that apply to all objects in the store. Not sure about it, but it appeared that these options only take effect on objects that were added after the options were set.

Often objects must be accessible for everyone, like static resources, images etc. for a website. So we want the whole world to have read access to our objects.

In Filezilla:

• click menu item Transfer > S3 Options > Canned ACL and check 'Public read access';
• click menu item Transfer > S3 Options > Storage class and check 'Standard'.

(Or choose other values where appropriate for your application.)

Now when a new file is added to the store these settings will always be applied to it.

Storage classes and ACLs (predefined sets of often needed permissions) are common S3 knowledge, widely available on the web.

3. Add an entry for the Object Store in FileZilla Site Manager

1. Click 'New Site', and give it a fancy name;
2. For Protocol select S3 - Amazon Simple Storage Service;
3. For Host enter a1234.upcloudobjects.com (NO dot up front, replace a1234 with your actual subdomain);
4. Select Logon Type 'Normal';
5. Enter Access Key ID and Secret Access key ID in their respective fields;
6. Click 'OK' to save.

Now next time when you select this connection (maybe approve the connection for the first time) and click 'Connect' you will see your buckets listed under 'Remote Site' and you can start adding files.

Hope it helps. As of this writing I couldn't find any resources.

It is very well possible that these instructions are not entirely correct or complete, so any suggestions, corrections or additions to this answer are more than welcome.