I just happen to have had this need in the past, and luckily saved a script just in case...
You'll need to change the strings on line 2 and 3.
Names Default To Here( 1 );
searchext = "jmp"; //extension to look for
searchfolder = "$DESKTOP"; //place to search
lstExtensions = {};
lstDates = {};
lstFiles = Files In Directory( searchfolder );
For Each( {i, j}, lstFiles,
lstExtensions[j] = Char( Right( i, 3 ) );
lstDates[j] = As Date( Creation Date( searchfolder || "/" || Char( i ) ) );
);
dt = New Table( "Files",
New Column( "Name", character, set values( lstFiles ) ),
New Column( "ext", character, set values( lstExtensions ) ),
New Column( "Date", numeric, set values( Matrix( lstDates ) ) )
);
dt << Select where( :ext != searchext );
dt << Delete Rows;
newest = Col Min( :Date );
newfile = searchfolder || "/" || :Name[dt << Get Rows Where( :Date == newest )][1];
Close( dt, no save );
Open( newfile );