UPD

class User < ActiveRecord::Base

  Paperclip.options[:swallow_stderr] = false 

  has_attached_file :photo, :styles => { :square => "100%", :large => "100%" },
    :convert_options => {
      :square => "-auto-orient -geometry 70X70#",
      :large => "-auto-orient -geometry X300" },
    :storage  => :s3,
    :s3_credentials => "#{RAILS_ROOT}/config/s3.yml",
    :path => ":attachment/:id/:style.:extension",
    :bucket => 'mybucket'

  validates_attachment_size :photo,
    :less_than => 5.megabyte

end

Works great on local machine, but gives me an error on Heroku: There was an error processing the thumbnail for stream.20143 The thing is I want to auto-orient photos before resizing, so they resized properly.

The only working variant now(thanks to jonnii) is resizing without auto-orient:

...
as_attached_file :photo, :styles => { :square => "70X70#", :large => "X300" },
        :storage  => :s3,
        :s3_credentials => "#{RAILS_ROOT}/config/s3.yml",
        :path => ":attachment/:id/:style.:extension",
        :bucket => 'mybucket'
...

How to pass additional convert options to paperclip on Heroku?