I know this post is a bit old, but this does not appear to work with the MVC widget. This returns an undefined variable
I found another post that mentioned updating the template to add an attribute. (https://www.telerik.com/forums/adding-an-id-to-selected-items) I have not tried this just yet, so I am not 100% certain this will work either.
Honestly, this seems to be a key feature missing from the widget. While I understand the widget wants to own the uid, this could cause a collision since the Guid is being generated on the client. A better option is to allow the server to manage the generation of the Uid and have it configurable to allow this.
This would handle two issues:
1) When you want to load existing files, you can set the key identifier that was generated when the file was uploaded. This would make the removal of the files in an edit screen a little easier.
2) You allow the server to manage the Uid which lowers the possibility of a collision associated with the Guid. In some cases, a developer may want to use a different identifier, for instance, a crypto-random-string. This would allow for them to do this. In some industries, this is a requirement. In my case, the Uid that is generated is only a temporary identifier and it is replaced when the file is moved to the location where it will be persisted.
For now, I hope the template works, but I highly recommend adding this to the backlog.