title: Grunt contrib-clean with files object error 'Object #<Object> has no method of 'indexOf' Use --force to continue
body: <p>I am using grunt contrib-clean to remove minified js files and want to specify the files by specifying the files object dynamically. When I do, I get the error: 'Object # has no method of 'indexOf' Use --force to continue.'</p>
  
  <p>Here are my options I am specifying.</p>
  
  <p><code>      </p>
  
  <pre><code>clean: {
          files: [{
              expand: true,
              cwd: 'js/',
              src: ['**/*.min.js', '!**/data/**', '!**/vendor/**', '!**/jquery/**', '!**/knockout/**'],
          }]
  },
  </code></pre>
  
  <p></code></p>
  
  <p>This pattern, without specifying the files object works, but I'm curious as to why the code above fails. 
  <code></p>
  
  <pre><code>    clean: {
          src: ['js/**/*.min.js', '!js/**/data/**', '!js/**/vendor/**', '!js/**/jquery/**', '!js/**/knockout/**'],
      },
  </code></pre>
  
  <p></code>
  I arrived at this problem when trying to use cwd for file operations because I have grunt installed in a sub-directory and was always trying to cwd to the root to perform operations, then I discovered <code>grunt.file.setBase('../')</code>. </p>
  
  <p>Any help would be appreciated. </p>
  
  <p>-greg</p>