[Rails] Active Storage how to validate file type

rails_active_storage_how_to_validate_file_type.md

Rails Active Storage how to restrict uploadable file types

Active Storage doesn't have validations yet.

We can restrict the accepted file types in the form:

<div class="field">
  <%= f.label :deliverable %>
  <%= f.file_field :deliverable, direct_upload: true, 
    accept: 'application/pdf, 
    application/zip,application/vnd.openxmlformats-officedocument.wordprocessingml.document' %>
 </div>

And add a custom validation in the model:

class Item
  has_one_attached :document

  validate :correct_document_mime_type

  private

  def correct_document_mime_type
    if document.attached? && !document.content_type.in?(%w(application/msword application/pdf))
      errors.add(:document, 'Must be a PDF or a DOC file')
    end
  end
end

Source: https://stackoverflow.com/questions/48349072/ruby-on-rails-active-storage-how-to-accept-only-pdf-and-doc?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa

@b-nik, Rails 6 has fixed the issue (Store newly-uploaded files on save rather than assignment). rails/rails@e8682c5

Of the two available gems I went with https://github.com/aki77/activestorage-validator as it is much simpler. You probably don't even need the gem as you could just bring in the validator class.