Jasny file upload postedfile always null - jasny-bootstrap

Question

Why is the PostedFile property always null even when a file is selected? The file upload appears to be working properly as far as allowing me select a file. But when the submit button is clicked that instantiates the code where the selected file is used, the PostedFile property is null.
This is my ascx code:
<div id="fileUploadDiv" class="row">
<div class="col-xs-12 push-left">
<div class="row">
<div class="col-xs-3 col-md-2 text-left form-group required label-wrapper">
<asp:Label
ID="Label1"
runat="server"
class="lblFileUpload"
Text="File to upload:"
CssClass="control-label"
EnableViewState="false" />
</div>
<div class="col-xs-9 col-xs-10 text-left">
<div class="fileinput fileinput-new input-group pull-left" data-provides="fileinput">
<div class="form-control" data-trigger="fileinput" style="max-width: 30em;">
<span class="fileinput-filename"></span>
</div>
<span class="btn btn-file">
<span class="fileinput-new">
<asp:Button
ID="Button4"
runat="server"
CssClass="button pink inline-narrow pull-right"
Text="Select File" />
</span>
<span class="fileinput-exists">
<asp:Button
ID="Button5"
runat="server"
CssClass="button pink inline-narrow pull-right"
Text="Change" />
</span>
<input
id="inputUploadFile"
runat="server"
type="file"
name="..."
accept=".jpg, .jpeg, .png, .eps, .pdf, .ai"
enctype="multipart/form-data">
</span>
<a href="#"
class="btn fileinput-exists"
data-dismiss="fileinput">
<asp:Button
ID="Button6"
runat="server"
CssClass="button pink inline-narrow pull-right"
Text="Remove" />
</a>
</div>
</div>
</div>
</div>
</div>
This is my c# code:
protected void btnAddUpdate_Click(object sender, EventArgs e)
{
...
byte[] fileData = null;
using (var binaryReader = new BinaryReader(inputUploadFile.PostedFile.InputStream)) //PostedFile is always null.
{
fileData = binaryReader.ReadBytes(inputUploadFile.PostedFile.ContentLength); //PostedFile is always null.
}
}

Answer 1

I found the issue. The file upload doesn't work asynchronously. The problem was solved by adding the following two lines of code to the OnInit method:
ScriptManager.GetCurrent(this.Page).RegisterPostBackControl(this.btnSave);
Page.Form.Attributes.Add("enctype", "multipart/form-data");
Where this.btnSave is the submit button on the page.