Passing file data from .Net backend to Javascript front-end

Question

I have a .Net (VB/C#) backend on one server and a javascript/jquery front-end on another. I am attempting to pass file data from the backend to the front-end by reading the file in .Net as a byte array, converting it to a base64 string and passing that to the js front-end along with a variable indicating the file type.

Once the data is in the js front-end I attempt to convert the base64 string to a file blob in order to read it with readAsDataURL() so that it can be displayed in an iframe or downloaded by the client.

For testing I am using a pdf file. The process of passing the data works but the file data is not recognized by the pdf viewer. The iframe loads the viewer but I get a message about the file not being recognized as a pdf file.

I have done a tone of searching and have gotten a lot of questions answered by searching stackoverflow but this eludes me.

Any suggestions would be much appreciated. Thank you.

Update: Here is my VB.Net code:

     Sub OnReadFileToViewer(ByVal filePath As String)
        Dim okToView As Boolean = False
        Dim fileBLOB As String = String.Empty
        Dim fileEncoded As String = String.Empty
        If (file.Exists(filePath)) Then
            Try
                Using fRead As FileStream = New FileStream(filePath, FileMode.Open, FileAccess.Read)
                    Dim bytes() As Byte = New Byte((CType(fRead.Length, Integer)) - 1) {}
                    Dim numBytesToRead As Integer = CType(fRead.Length, Integer)
                    Dim numBytesRead As Integer = 0

                    While (numBytesToRead > 0)
                        ' Read may return anything from 0 to numBytesToRead.
                        Dim n As Integer = fRead.Read(bytes, numBytesRead, _
                            numBytesToRead)
                        ' Break when the end of the file is reached.
                        If (n = 0) Then
                            Exit While
                        End If
                        numBytesRead = (numBytesRead + n)
                        numBytesToRead = (numBytesToRead - n)

                    End While
                    numBytesToRead = bytes.Length
                    fileEncoded = System.Convert.ToBase64String(bytes)
                End Using
                okToView = True
            Catch ex As Exception
                '
            End Try
        End If

        If (okToView) Then
            ' Code to send data to js fron-end
        End If

    End Sub

And my js front-end for converting the base64 string to BLOB and the iframe preview:

    function PreviewFile(file) {
      var fileBlob = b64toBlob(file,'application/pdf','');
      var reader = new FileReader();
      var fileType = e.target.getAttribute("type");
      alert(fileType);
      reader.onload = function (e) {
         if (fileType)
           $("#fileViewerDiv").append('<iframe id="fileVieweriFrame" class="fileViewer"></iframe>'); // + e.target.result + '">');
           $("#fileVieweriFrame").attr('src', file); //e.target.result);
           $("#fileVieweriFrame").dialog();
          }
     reader.readAsDataURL(fileBlob);
    }

    function b64toBlob(b64Data, contentType, sliceSize) {
      contentType = contentType || '';
      sliceSize = sliceSize || 512;

      var byteCharacters = atob(b64Data);
      var byteArrays = [];

      for (var offset = 0; offset < byteCharacters.length; offset += sliceSize) {
          var slice = byteCharacters.slice(offset, offset + sliceSize);

          var byteNumbers = new Array(slice.length);
          for (var i = 0; i < slice.length; i++) {
             byteNumbers[i] = slice.charCodeAt(i);
          }

          var byteArray = new Uint8Array(byteNumbers);

          byteArrays.push(byteArray);
       }

      var blob = new Blob(byteArrays, {type: contentType});
      return blob;
 }

The alert(fileType) does display the correct file type - application/pdf.

I have to admit that I lifted the b64toBlob() function from a blog somewhere but I have forgotten which one and have lost the URL to the site. My apologies to the original author. If anyone recognizes the code and knows the author/site please let me know

Edit b64toBlob() by Jeremy Banks - Based on this SO answer