Benito Serna Tips and tools for Ruby on Rails developers

How to attach an active storage file to an email

January 27, 2021

Are you using Active Storage to handle file uploading to the cloud (maybe AWS) and you want to know how to attach an uploaded file to an email?… But you are not familiar with the Active Storage and Action Mailer APIs?

I answered this question on reddit some days ago =)… Here is how you can do it…

You need to download the file and assign as and attachment, something like this…

attachments[photo.filename.to_s] =

Here the context of the example…

class Product < ApplicationRecord
  has_one_attached :photo

class PhotoMailer < ApplicationMailer
  default from: ""

  def new_photo_email
    photo = params[:product].photo
    attachments[photo.filename.to_s] =
    mail(to: "", subject: "Nueva photo")

class PhotoMailerPreview < ActionMailer::Preview
  def new_photo_email
    PhotoMailer.with(product: Product.first).new_photo_email

Here the reference from the rails guides

Why using “download” works?

If you are not familiar with Action Mailer and you are reading the docs, maybe is not that clear what you need to do. Here is what I understand.

On the Action Mailer docs you can see…

attachments['filename.jpg'] ='/path/to/filename.jpg')

Inside the [] you need to put the name that you want for the attachment and you need to assign the content of the file. Like this:

attachments[filename] = file_content

In the example from the docs, returns the content of the file.

Now, in the ActiveStorage docs says that you can “Use the attachment’s download method to read a blob’s binary data into memory”, and they use this example:

binary =

That’s why I propose you to use download. Apparently download will return the content of the file wherever the file is stored.

Related articles

Weekly tips and tools for Ruby on Rails developers

I send an email each week, trying to share knowledge and fixes to common problems and struggles for ruby on rails developers, like How to fetch the latest-N-of-each record or How to test that an specific mail was sent or a Capybara cheatsheet. You can see more examples on Most recent posts or All post by topic.