You can create a formula, but I would go for a function in VBA instead.

You can do something like:

Function splitName(text As String) As String
txt = Split(text, "\")
splitName = txt(UBound(txt))
End Function

Which will return "signature.htm". Add the function to a module so that it
is available in the worksheet (or create an addin with the function)

Regards
Robert