title: R markdown error: can't produce HTML file
body: <p>I can't produce an HTML document from any .Rmd file by pressing the Knit HTML button in R Studio, including the sample document that summarizes and plots the cars dataset. Here's the error message that I get:</p>
  
  <pre><code>Error: unrecognized fields specified in html_dependency: attachment
  Execution halted
  </code></pre>
  
  <p>However, I can produce the HTML document by running the command <code>knit2html("file.Rmd")</code>. The Knit PDF button also functions properly. Does anyone have an idea about how to fix this error?</p>
  
  <p>Here's the sample .Rmd file that I have:</p>
  
  <pre><code>title: "Untitled"
  author: "blakeoft"
  date: "Friday, September 12, 2014"
  output: html_document
  ---
  
  A summary:
  
  ```{r}
  summary(cars)
  ```
  </code></pre>