title: download.file() fails when appending a random suffix to the filename
body: <p>I'm trying to download a file in R on a remote server which sits behind a number of proxies. Something - I can't figure out what - is causing the file to be returned cached whenever I try and access it on that server, whether I do so through R or just through a Web Browser.</p>
  
  <p>I've tried using <code>cacheOK=FALSE</code> in my <code>download.file</code> call and this has had no effect.</p>
  
  <p>Per <a href="https://stackoverflow.com/questions/1431512/is-there-a-way-to-force-browsers-to-refresh-download-images">Is there a way to force browsers to refresh/download images?</a> I have tried adding a random suffix to the end of the URL:</p>
  
  <pre><code>download.file(url = paste("http://mba.tuck.dartmouth.edu/pages/faculty/ken.french/ftp/F-F_Research_Data_Factors_daily.zip?",
                            format(Sys.time(), "%d%m%Y"),sep=""), 
                destfile = "F-F_Research_Data_Factors_daily.zip", cacheOK=FALSE)
  </code></pre>
  
  <p>This produces, e.g., the following URL:</p>
  
  <pre><code>http://mba.tuck.dartmouth.edu/pages/faculty/ken.french/ftp/F-F_Research_Data_Factors_daily.zip?17092012
  </code></pre>
  
  <p>Which when accessed from a Web Browser on the remote server, indeed returns the latest version of the file. However, when accessed using <code>download.file</code> in R, this returns a corrupted zip archive. Both WinRAR and R's <code>unzip</code> function complain that the zip file is corrupt.</p>
  
  <pre><code>unzip("F-F_Research_Data_Factors_daily.zip")
  1: In unzip("F-F_Research_Data_Factors_daily.zip") :
  internal error in unz code
  </code></pre>
  
  <p>I can't see why downloading this file via R would cause a corrupted file to be returned, whereas downloading it via a Web Browser gives no problem.</p>
  
  <p>Can anyone suggest either a way to beat the cache from R (about which I'm not hopeful), or a reason why <code>download.file</code> doesn't like my URL with ?someRandomString tacked onto the end of it?</p>