Can't upload PDF using carrierwave on Heroku?

In my rails 4 app, I'm using carrierwave to upload files to google cloud storage. I'm able to successfully upload image files, but pdfs are not working. It shows following error:

You are not allowed to upload "pdf" files, allowed types: jpg, jpeg, gif, png

Here is my uploader:-

# encoding: utf-8
require 'carrierwave/processing/mime_types'
class AttachmentUploader < CarrierWave::Uploader::Base

  include CarrierWave::MiniMagick
  include CarrierWave::MimeTypes

  storage :fog

  def store_dir
    "uploads/#{model.class.to_s.underscore}/#{mounted_as}/#{model.id}"
  end

  process :set_content_type

  def extension_white_list
    %w(jpg jpeg gif png pdf)
  end

end

I don't know what's wrong with this code.

And here's the attachment model

class Attachment < ActiveRecord::Base
  mount_uploader :attachment, AttachmentUploader

  # Associations

  belongs_to :attached_item, polymorphic: true

  # Validations

  validates_presence_of :attachment
  validates_integrity_of :attachment

  # Callbacks

  before_save :update_attachment_attributes

  # Delegate

  delegate :url, :size, :path, to: :attachment

  # Virtual attributes

  alias_attribute :filename, :original_filename

  private

  def update_attachment_attributes
    if attachment.present? && attachment_changed?
      self.original_filename = attachment.file.original_filename
      self.content_type = attachment.file.content_type
    end
  end
end

Update: When testing on local machine it works but not on heroku. On development env it works without issue and I can check file uploaded to google storage. But on heroku it displays error "You are not allowed to upload "pdf" files, allowed types: jpg, jpeg, gif, png"

Thanks in advance!

via Chebli Mohamed