Changing Mime type on loaded files

I can see how to set the Mime type for a certain file extension before
loading files into a Repository Workspace.

(Window->Preferences/Jazz Source Control->File Properties)

I've loaded an entire source tree with files containing an unusual
extension. These files are all text, but because that extension isn't
defined above as a Text file, they've all been loaded as binary.

How can I change the Mime type on all files matching a given pattern
after they've been loaded ??

Thanks