Relax Paperclip dependency to allow 4.x
Explicitly skip attachment file type validation

Starting with Paperclip version 4.0, all attachments are required to include a content_type validation, a file_name validation, or to explicitly state that they're not going to have either. Paperclip raises MissingRequiredValidatorError error if you do not do this.
Guard against older versions of Paperclip
Merge pull request #8 from Sija/patch-1

Update tests for Paperclip 4.x compatibility

thanks @Sija