I am trying to upload a file using selenium web driver. The UI has an upload button which is inside a div tag. The button has a hidden textbox which is inside a span tag. I am able to locate till the span tag.

<div class="ng-scope" ng-if="showUpload" style="padding-bottom:5px;">
<div class="btn ng-binding" ng-class="class" flow-drop="" style="padding-top:30px;vertical-align:middle;width:370px;height:100px;background-color:#eee;border:1px solid #ccc" ng-style="style" flow-drag-leave="style={border:'1px solid #ccc'}" flow-drag-enter="style={border:'1px solid #007670'}" flow-prevent-drop="">Drag and Drop your files here.</div>
<span class="ng-binding" style="padding:0 20px">or</span>
<span class="btn btn-primary ng-binding" flow-attrs="{accept:'image/*,.pdf,.csv,.txt,.doc'}" flow-btn="">
<i class="icon icon-file"/>
Upload File
<input type="file" style="visibility: hidden; position: absolute; width: 1px; height: 1px;" multiple="multiple" accept="image/*,.pdf,.csv,.txt,.doc"/>
</span>
</div>

I am able to locate the input tag by firepath, but the same using java code doesnt work.

WebElement m_upload = driver.findElement(By.CssSelector("input[type='files']")); returns null. Can anyone tell me how to go about this?

try following code

//Give your file path here
  var orgFile = @"C:\Users\username\Downloads\pdf.pdf";
//get uploadfile input and send pat using SendKeys
            var uploadFile = driver.FindElement(By.CssSelector("input[type='file']"));
            uploadFile.SendKeys(orgFile);

if any issue then let me know.