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.
    #
    # Note: This depends on the patch from https://github.com/mikel/mail/pull/858, which adds
    # parts.inspect_structure and fixes an issue with mail.without_attachments!
    #
    # Until that's merged in, you can use:
    # For Rails 4: gem 'mail', github: 'TylerRick/mail', branch: 'without_attachments.2'
    # For Rails 3: gem 'mail', github: 'TylerRick/mail', branch: 'without_attachments_rails_3.2'
    #
    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

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

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

      # Log the structure of the email (which parts are within which other parts), including
      # attachments
      payload[:mail] +=
        divider_line +
        "Structure:\n" +
        original_structure + "\n\n"

      payload[:mail] +=
        divider_line +
        "Structure without attachments:\n" +
        mail.inspect_structure + "\n\n" if attachments.any?

      # 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 {|part|
          next if part.parts.any?

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

Note: This depends on the patch from mikel/mail#858, which adds parts.inspect_structure and fixes an issue with mail.without_attachments!

See problem as discussed here: http://stackoverflow.com/questions/28776785/how-do-i-get-actionmailer-to-log-sent-messages-but-not-include-the-attachments