If user types "tt:tt" in the SaveAs dialog it is treated as an absolut URL.
After discussion with SBA and MBA there seems to be the following ways to handle
the situation:

1) Leave it as it is, means if a string containing ':' is inserted in SaveAs
dialog it is treated as absolut URL. So the office is not able to generate files
with such names. 

2) Let the dialog check whether there is a content provider related to the
substring recognized as a protocol name, if there is no and the system accepts
the string as a filename treat it as a file name.

The inconvinience of the solution (1) is that if user inserts the same string in
the OpenFile dialog, it is treated as a file name if such file exists. From
other side usage of this symbol in a file name is not acceptable on Windows
system, and very unusual on Unix systems.
The second solution fits to the case of SaveAs dialog since it makes no sence to
store a file to the URL that is not supported by UCB. The problem is that for
the same string the behaviour will be different depending from whether the user
has content provider registered for the protocol or not. From other side the
optional content provider seems to be a very rare case as well.