Overriding content_type for Rails Paperclip plugin

I think I have a bit of a chicken and egg problem. I would like to set the content_type of a file uploaded via Paperclip. The problem is that the default content_type is only based on extension, but I'd like to base it on another module.

I seem to be able to set the content_type with the before_post_process

class Upload < ActiveRecord::Base 
  has_attached_file :upload
  before_post_process :foo

  def foo
    logger.debug "Changing content_type"
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    #This works
    self.upload.instance_write(:content_type,"foobar")

    # This fails because the file does not actually exist yet
    self.upload.instance_write(:content_type,file_type(self.upload.path)
  end

  # Returns the filetype based on file command (assume it works)
  def file_type(path)
    return `file -ib '#{path}'`.split(/;/)[0]
  end
end

But...I cannot base the content type on the file because Paperclip doesn't write the file until after_create.

And I cannot seem to set the content_type after it has been saved or with the after_create callback (even back in the controller)

So I would like to know if I can somehow get access to the actual file object (assume there are no processors doing anything to the original file) before it is saved, so that I can run the file_type command on that. Or is there a way to modify the content_type after the objects have been created.

---

Probably you could use upload.to_file. It gives you paperclip temporary file (Paperclip::Tempfile). It has path property, so you can use

self.upload.instance_write(:content_type,file_type(self.upload.to_file.path)

You can get Tempfile using upload.to_file.to_tempfile