Why do I get a string encoding issue “\\xE2” from

I'm trying to download a PDF from an email and write the contents to a file. For some reason, I'm getting this error:

An Encoding::UndefinedConversionError occurred in attachments#inbound: "\xE2" from ASCII-8BIT to UTF-8 app/controllers/api/attachments_controller.rb:70:in `write'

Here's my code:

def inbound
    if Rails.env.production? or Rails.env.staging?
      email = Postmark::Mitt.new(request.body.read)
    else
      email = Postmark::Mitt.new(File.binread "#{Rails.root}/app/temp_pdfs/email.json")
    end

    if email.attachments.count == 0
      # notify aidin that we got an inbound email with no attachments
      respond_to do |format|
        format.json { head :no_content }
      end
      return
    end
    attachment = email.attachments.first
    filename = "attachment" + (Time.now.strftime("%Y%m%d%H%M%S")+(rand * 1000000).round.to_s) + ".pdf"
    base_path = "#{Rails.root}/temp_attachments/"
    unless File.directory?(base_path)
      Dir::mkdir(base_path)
    end
    file = File.new base_path + filename, 'w+'
    file.write Base64.decode64(attachment.source['Content'].encode("UTF-16BE", :invalid=>:replace, :replace=>"?").encode("UTF-8"))
    file.close
    write_options = write_options()
    write_options[:metadata] = {:filename => attachment.file_name, :content_type => attachment.content_type, :size => attachment.size }

    obj = s3_object()
    file = File.open file.path
    obj.write(file.read, write_options)
    file.close

    FaxAttach.trigger obj.key.split('/').last

    render :nothing => true, :status => 202 and return
  end

I read around and it looked like the way to solve this was:

file.write Base64.decode64(attachment.source['Content'].encode("UTF-16BE", :invalid=>:replace, :replace=>"?").encode("UTF-8"))

but it doesn't seem to work.

The error message is actually being thrown on the file write, not by your encode/decode inside the params, because Ruby is trying to apply default character encoding on file.write. To prevent this, the quickest fix is to add the b flag when you open the file

file = File.new base_path + filename, 'wb+'
file.write Base64.decode64( attachment.source['Content'] )

That's assuming the incoming attachment is encoded in Base64, as your code implies (I have no way to verify this). The Base64 encoding stored inside attachment.source['Content'] should be the same bytes in ASCII-8BIT and UTF-8, so there is no point converting it inside the call to decode64.