Example for updating metadata of S3 objects with fog.

update_s3_metadata.rb

require 'fog'

access_key_id = ENV['AWS_ACCESS_KEY_ID']
secret_access_key = ENV['AWS_SECRET_ACCESS_KEY']
region = ENV['S3_REGION']  # eg: 'us-east-1'
bucket_name = 'mybucket'

target_object = 'object_filename'  # key for S3 object in bucket
metadata_key = 'Content-Type'  # Metadata key to set
metadata_value = 'text/html'  # Metadata value to set

storage = Fog::Storage.new(
  provider: 'AWS',
  aws_access_key_id: access_key_id,
  aws_secret_access_key: secret_access_key,
  region: region
)
bucket = storage.directories.get(bucket_name)

file = bucket.files.find { |i| i.key == target_key }
file.reload
file.metadata = { metadata_key => metadata_value }
file.save