Thanks for using Syncfusion Products.
The root cause of the issue “Two buttons are shown in uploadbox. The top button does not do anything at all, while the lower button opens the File selection dialog as expected.” is syncfusion theme files doesn’t loaded properly in your sample. When you access the design page of uplaodbox control, it will include our necessary script and css files as embedded resource from the EJ.Web assembly itself by adding the following key in web.config file. So, please ensure that the themes are set as true which is included in web.config file.
<add key="LoadEJResourcesFromAssembly" value="true" />
<add key="EJResources" value="jsrender:true;jqueryeasing:true;globalize:true;themes:true;" />
If you are going to refer scripts and CSS externally, refer the necessary scripts and CSS files in Site.Master page and set false to the“LoadEJResourcesFromAssembly $ EJResources” key which is in the web.config file, it will skip the scripts and CSS files reference from assembly.
<add key="LoadEJResourcesFromAssembly" value="false" />
<add key="EJResources" value="jsrender:false;jqueryeasing:false;globalize:false;themes:false;" />
Also, we have prepared the simple sample to render the UploadBox component and it is available from the following location:
We have showcased the getting started to render our component by using embedded resource in the specified UG Documentation link,
Can you please check the above sample? If still you face the problem, please revert us by modifying the sample based on your application along with replication procedure. This would be helpful for us to serve you.
Please let us know if you have further query.