The problem is described in this code library
The repro markup can be found below
<!DOCTYPE html>
<
html
xmlns
=
"http://www.w3.org/1999/xhtml"
>
<
head
runat
=
"server"
>
<
title
></
title
>
<!-- Bootstrap CSS -->
<
link
rel
=
"stylesheet"
href
=
"https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css"
integrity
=
"sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm"
crossorigin
=
"anonymous"
>
<!-- Optional JavaScript -->
<!-- jQuery first, then Popper.js, then Bootstrap JS -->
<
script
src
=
"https://code.jquery.com/jquery-3.2.1.slim.min.js"
integrity
=
"sha384-KJ3o2DKtIkvYIK3UENzmM7KCkRr/rE9/Qpg6aAZGJwFDMVNA/GpGFF93hXpG5KkN"
crossorigin
=
"anonymous"
></
script
>
<
script
src
=
"https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.12.9/umd/popper.min.js"
integrity
=
"sha384-ApNbgh9B+Y1QKtv3Rn7W3mgPxhU9K/ScQsAP7hUibX39j7fakFPskvXusvfa0b4Q"
crossorigin
=
"anonymous"
></
script
>
<
script
src
=
"https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/js/bootstrap.min.js"
integrity
=
"sha384-JZR6Spejh4U02d8jOt6vLEHfe/JQGiRRSQQxSfFWpi1MquVdAyjUar5+76PVCmYl"
crossorigin
=
"anonymous"
></
script
>
</
head
>
<
body
>
<
form
id
=
"form1"
runat
=
"server"
>
<
asp:ScriptManager
runat
=
"server"
/>
<
telerik:RadAsyncUpload
ID
=
"RadAsyncUpload1"
runat
=
"server"
Skin
=
"Bootstrap"
RenderMode
=
"Lightweight"
></
telerik:RadAsyncUpload
>
</
form
>
</
body
>
</
html
>