Hi James,
I am afraid that Upload Progress control doesn't have any property to set progress text and size of the progress text. I will forward this to the development team and update you tomorrow on this regard.
It doesn't allow to display any message when an invalid extension is passed instead it supports to allow/deny the required file extension while uploading. It can be performed by setting the ExtensionsAllow and ExtensionsDeny property of UploadBox.
I have attached a sample, which explains the usage of ExtensionsAllow and ExtensionsDeny property.
Attached Sample :
UploadProgress.zipKindly let me know if this helps you.
Thank you for using Syncfusion Products.
Thanks,
Maheswari