Feed an .csv file up to server for importing data, then throw away?

I've written a routine allowing the client user to declare where
their .csv import file resides and then send it to the server to
import the many rows into an existing model table, and then throw it
away.
I've imported many graphic files into my application using
attachment_fu and this was very straightforward as they were imported
into an existing model object. Instead, I am trying to import a file,
that when it's processed will be 'thrown away'. These examples are
running ok, but I won't be able to use this strategy in my production
environment...
Here's my view code:

<%= form_tag(:url => {:action=> "post" }, :html => { :multipart =>
true }) -%>
<p><%= file_field_tag "file", :size => 75, :class => "csv-input" -%></

<p><strong><label for="file">File to Upload</label></strong></p>
<p><%= submit_tag -%></p>

Here's my controller code:

class ImportController < ApplicationController
  def aaccount_import
    @owner = Owner.find(2)
    n = 0
    unless params[:file].nil?
      fullpath = File.expand_path(params[:file]) # this doesn't blow
up but some books say that a file object is passed and thus File
methods should apply?
      FasterCSV.foreach("#{RAILS_ROOT}/convert/#{params[:file]}") do |
row>
# FasterCSV.foreach(params[:file]) do |row| # cant seem to read
submitting machines full path info?
# FasterCSV.foreach(fullpath) do |row|
        c = @owner.accounts.new
        c.code = row[0]
        c.description = row[1]
        if c.save
          n += 1
        else
          n += 1
          d = Badboy.new
          d.message = 'Aaccount Record Failed to convert'
          d.recno = n
          d.save
        end
      end
    end
  end
end

You'll notice that I am able to import this file on MY OWN machine but
I'm having to HARD CODE the file path before the actual file name that
is delivered. You can see some ideas I've tried to make the
params[:file] behave like a FILE object and thus reveal its full path.
I am grateful for any links or suggestions.
Kathleen

params[:file] will either be a StringIO or an actual instance of File (not a file path or anything like that)

FasterCSV.new(params[:file]).each ... probably does what you want

Fred