You have at least 5 options:

• Stanislav Vitvitskiy's solution
• RestClient

require 'rest_client'
RestClient.post 'http://localhost:3000/foo', fields_hash.merge(:file => File.new('/path/to/file'))

• The curb gem
• The multipart-post Net:HTTP extension
• Calling curl from Ruby with, for example, Open3.

Open3.popen3('curl  <and your parameters>') do |input, output, error|
# do something
end