title: Sitecore Pipeline Upload Processor
body: <p>I'm using UploadProcessor to block specific file uploading into MediaLibrary.
  Everything is working good and I can see the alert Sitecore's message. But, Sitecore's error message is not really user-friendly <b>"One or more files could not be uploaded. See the Log file for more details"</b></p>
  
  <p>So, I'd like to add extra alert box for users. Below is my code, but the javascript is not working.</p>
  
  <p><img src="https://i.stack.imgur.com/BFja7.png" alt="enter image description here"></p>
  
  <p>Some people want me to use "SheerResponse", but Sitecore Document mentions that</p>
  
  <blockquote>
    <p>The uiUpload pipeline is run not as part of the Sheer event, but as part of the form loading process in response to a post back. This is because the uploaded files are only available during the “real” post back, and not during a Sheer UI event. In this sense, the uiUpload pipeline has not been designed to provide UI. In order to provide feedback to a User, the processor should resort to some trick which emits the JScript code.
    <a href="http://sdn.sitecore.net/Articles/Media/Prevent%20Files%20from%20Uploading/Pipeline%20upload.aspx" rel="nofollow noreferrer">http://sdn.sitecore.net/Articles/Media/Prevent%20Files%20from%20Uploading/Pipeline%20upload.aspx</a></p>
  </blockquote>
  
  <p>Do you have any idea how to implement alert box??</p>