Override the version from the actionmailer gem in order to not log attachments.

action_mailer-dont_log_attachments.rb

ActionMailer::Base.class_eval do
  class << self
    protected

    # Override the version from the actionmailer gem in order to not log attachments.
    #
    def set_payload_for_mail(payload, mail) #:nodoc:
      payload[:mailer]     = name
      payload[:message_id] = mail.message_id
      payload[:subject]    = mail.subject
      payload[:to]         = mail.to
      payload[:from]       = mail.from
      payload[:bcc]        = mail.bcc if mail.bcc.present?
      payload[:cc]         = mail.cc  if mail.cc.present?
      payload[:date]       = mail.date

      # Don't include attachments in what is logged because it causes the log file to grow too
      # quickly and is unreadable by a human reading the log file. Really the main things that
      # would be useful to a developer looking back at the logs to debug a problem or to confirm
      # that things went out as expected are: 1. The headers. 2. The message body (text and HTML).
      # 3. Which files were attached (file name and type should be sufficient).
      # If you want to see the attachments themselves, just add a mail interceptor that bcc's
      # outgoing mail to your inbox.

      # Copy the mail by re-parsing; self.dup does not create copies of the parts, so deleting attachments
      # would also delete them from the original mail object.
      mail = Mail.new(mail.encoded)
      # mail = mail.dup

      mail.without_attachments!
      divider_line = '-' * 80 + "\n"

      payload[:mail] = mail.header.encoded + "\n\n"

      # Add a searchable, human-readable version of the body parts. Since the encoded version uses
      # fixed line length, it may break a line at any point, even the middle of a word. So if you
      # later go and grep for the word, you may not find it if all you have is the encoded version,
      # even if the word is present! Calling mail.decoded gives an error, so we have to loop
      # through the parts individually.
      payload[:mail] +=
        divider_line +
        "Decoded parts:\n" +
        mail.parts.to_enum(:recursive_each).map do |part|
          next if part.parts.any?

          divider_line +
            part.inspect + "\n" +
            part.decoded.force_encoding('utf-8')
        end.join("\n") +
        '-' * 80 + "\n\n" rescue $!.inspect
    end
  end
end