Error while importing logon par file

Hi

I have to do logon page customisation for that i have taken com.sap.portal.runtime.logon.par.bak file from basis team and i saved it to my desktop .Now i have renamed it to com.sap.portal.runtime.logon.par and trying to import to my NWDS.

So i opend EP perspective file->import->browsed the par file in place of Archive file name when i click on finish it is showing the error "invalid END header(bad central directory offset).

"The plugin com.sap.portal.plugins.config-archiver"caused an exception during the unpack operation.see log for more details"

Can anybody please let me know the reason for this error .

Thanks in advance.