Rails using RestClient to call mailgun with attachment. When I was searching I just found same ways using RestClient::Request.execute, so I'm sharing this other way

Rails-RestClient-with-mailgun.rb

require 'json'

result = RestClient.post "https://api:[email protected]/v2/domain.com/",
:from => "YourDomain <[email protected]>",
:to => "[email protected]",
:subject => "Mailgun with attachment",
:text => "Hello, Put something here",
:multipart => true,
:attachment => File.new('path/to/file', 'rb')