I created a test case that had in its design a series of files to run as attachments.  I then used the "create manual test script from test design" to create a script from these.  This all worked great, but now I want to update a few of the test files.  Is there any way to do that in both places at once - I basically want to replace the old attachment with a new attachment.

I tried reuploading the attachment from the attachments page, and got a message that said:
An attachment with the selected file name is already to be added when this work item is saved <filename>
With an OK button.

That makes it sound like it will be updated, but it isn't.

Is there some way to accomplish what I want?  Hopefully an easy way?

Or perhaps there is a better way to use the system?