```ruby
class Document < ActiveRecord::Base
  belongs_to :user
  has_attached_file :doc_pdf, styles: {:thumb => "100x100#",
      :small  => "150x150>",
      :medium => "200x200" }

  validates_attachment_content_type :doc_pdf, content_type: ['image/jpeg', 'image/png', 'image/gif', 'application/pdf']
end
```

Remove attachment styles and unnecessary content type
user
```ruby
class Document < ActiveRecord::Base
  belongs_to :user
  has_attached_file :doc_pdf

  validates_attachment_content_type :doc_pdf, content_type: "application/pdf"
end

```
assistant