When you have string contents (e.g. a generated binary stream, or data from a remote source) that you want to store as a file using Carrierwave, here is a simple solution.

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While you could write your string to a file and pass that file to Carrierwave, why even bother? You already have your string (or stream).
However, a plain StringIO object will not work for Carrierwave's ActiveRecord integration:

>> Attachment.create!(file: StringIO.new(contents))
TypeError: no implicit conversion of nil into String

This is because Carrierwave expects a filename by default. It makes sense, so here is a simple yet useful helper class:

class FileIO < StringIO
  def initialize(stream, filename)
    super(stream)
    @original_filename = filename
  end

  attr_reader :original_filename
end

You must simply come up with a filename and can then pass your string or stream directly.

>> Attachment.create!(file: FileIO.new(contents, 'document.pdf'))
=> #<Attachment ...>

When using zeitwerk (Rails 6) you might also need to add an inflection in order to make autoloading work for this file:

ActiveSupport::Inflector.inflections(:en) do |inflect|
  inflect.acronym 'IO'
end

Otherwise zeitwerk will complain that the file file_io.rb does not define FileIo.