So the correct configuration to use this S3 storage is:
Use SSL: No
Storage type: S3 compatible
Server: custom URL (s3.fr-par.scw.cloud)
Bucket name: bucket name
Region: Custom region value (fr-par)
Storage class: Standard
Folder path: folder path
AWS Access ID: (generated access key)
AWS Access Key: (generated secret key)
Client library to use: Amazon AWS SDK
Advanced option [s3-ext-authenticationregion = ‘fr-par’]