We now use an "Image" entity type defined in test/data/schema.py with a
"data" bytes attribute along with a "thumbnail" one. The former is used
as File's data attribute was. The latter is now used within
S3StorageMigrationTC (instead of "data" attribute); this is in
preparation of the next changeset in which s3 storage setup will be
handled in a test hook (instead of test setup).