Would you please provide more details on the Bootstrap version you are using and also the markup of the AsyncUpload? Also, please check if setting the RenderMode of the AsyncUpload to Lightweight would make any difference.
Meanwhile, you can follow the suggestions in the first two points of the Improve Your Debugging Skills with Chrome DevTools
blog post to inspect the applied styles that distort the correct appearance.
Once you know the styles you need to override, you can use the same style selector and add "html body " in front of it to make it more specific, "stronger". More on CSS specificity you can find here:
Get quickly onboarded and successful
with your Telerik and/or Kendo UI products with the Virtual Classroom free technical training, available to all active customers. Learn More.