Setting the CancelButtonStyle has no effect whether done in the markup or code-behind. When the button is rendered to the page, it does include the correct id. I am using version 4.0 2015-12-29. This worked correctly in version 3.0.

Markup:

<div id="UploadContainer" style="width:290px">

<CuteWebUI:UploadAttachments ID="Attachments1" runat="server" 

InsertButtonID="UploadPhotos" QueuePanelID="UploadContainer" ProgressCtrlID="UploadContainer"

ShowFileIcons="true" ShowCheckBoxes="false"

ShowActionButtons="false" ShowRemoveButtons="false"

ShowProgressInfo="false" MultipleFilesUpload="true"

ProgressPanelWidth="250" ProgressLabelStyle-Width="200"

AttachmentTRStyle="Width:0px" NumFilesShowCancelAll="9999"

OnAttachmentAdded="Attachments1_AttachmentAdded"

CancelUploadMsg="Cancel" CancelButtonStyle-CssClass="inputbutton"

>

<HeaderRowStyle Width="0px" />

<ValidateOption AllowedFileExtensions="jpeg,jpg,gif,png" MaxSizeKB="5120" />

</CuteWebUI:UploadAttachments>

</div>

Rendered content:

<div id="UploadContainer" style="width: 290px; display: none;">
   <style>...</style>
<input name="ctl00$Body$Attachments1" id="ctl00_Body_Attachments1" type="hidden" value="" autocomplete="off" IsUploaderField="1"><span style="display: none;"></span>

<button style="display: none;">Cancel</button>

<img id="ctl00_Body_Attachments1Image_unique" style="display: none;" onerror="this.onload()" onload="this.style.display=&quot;none&quot; ; if(window.CuteWebUI_AjaxUploader_Initialize){window.CuteWebUI_AjaxUploader_Initialize(this.id)}else{var imgid=this.id;var xh=window.XMLHttpRequest?new XMLHttpRequest():new ActiveXObject('Microsoft.XMLHttp');xh.open('GET','/fullauction/CuteWebUI_Uploader_Resource.axd?type=script&amp;_ver=635899698420000000',true);xh.onreadystatechange=function(){if(xh.readyState==4){eval(xh.responseText);CuteWebUI_AjaxUploader_Initialize(imgid);}};xh.send('');}" src="/fullauction/CuteWebUI_Uploader_Resource.axd?type=file&amp;file=continuous.gif" resourcehandler="/fullauction/CuteWebUI_Uploader_Resource.axd" serverlang="AspNet" contextvalue="Fr!3lvP0C0IUpF2THIuRj!1rMIDrm4YNcifofyb!1VDBmqk2qAc3wsxn2idku1nEi4yeqtg8SuQYkKSsYqj2kQXWWqvAziBOH8iQexNJgtEQWs!2" inputboxcsstext="" inserttext="Upload multiple files" extensions="jpeg,jpg,gif,png" maxsizekb="5120" queuepanelid="UploadContainer" progressctrlid="UploadContainer" progresstextid="ctl00_Body_Attachments1__ProgressText" cancelbuttonid="ctl00_Body_Attachments1__Cancel"  insertbuttonid="UploadPhotos" postbackeventreference="__doPostBack('ctl00$Body$Attachments1','')" border barstyle="Continuous" infostyle="padding-left:3px;font:normal 12px Tahoma;" barheight="20" panelwidth="250" showprogressbar="1" showqueuetable="1" numfilesshowcancelall="9999" multiplefilesupload="1" uploadcursor="Auto" uploadtype="Auto" advancedoptionnosilverlight="true" uploadaddonbuttonmode="Auto" uploadingmsg="Uploading.." cancelallmsg="Cancel all Uploads" canceluploadmsg="Cancel" windowsdialoglimitmsg="Unable to select so many files at once. The total file name length cannot exceed 32kb." maxfileslimitmsg="The maximum number of files allowed to be uploaded is set to {0}." filetoolargemsg="{0} cannot be uploaded!&#10;&#10;File size ({1}) is too large. The maximum file size allowed is set to: {2}." verticks="635899698420000000" root="/fullauction/" namespace="CuteWebUI" uniqueid="ctl00$Body$Attachments1">
  </div>