how to get column's last value in a Temporary datatable (.net)

I assume you're talking about a System.Data.DataTable ..?

If the last row always has the highest ID then simply get the last row of the table:

DataRow lastRow = yourDataTable.AsEnumerable().Last();

int id = lastRow.Field<int>("ID"); // maybe you have to change "int" to the actual column type here
string fileName = lastRow.Field<string>("FILENAME");
string path = lastRow.Field<string>("PATH");

If the last row is not neccessarily the one with the highest ID and you have not defined the ID-column as primary key, then:

int highestID = yourDataTable.AsEnumberable()
                             .Select(row => row.Field<int>("ID"))
                             .Max(); // above comment about "int" applies here too

DataRow rowWithHighestID = yourDataTable.AsEnumberable()
                             .Single(row => row.Field<int>("ID") == highestID);

If you do have defined the ID-column as primary key, then:

int highestID = yourDataTable.AsEnumberable()
                             .Select(row => row.Field<int>("ID"))
                             .Max(); // above comment about "int" applies here too

DataRow rowWithHighestID = yourDataTable.Rows.Find(highestID);

Edit:

The above "middle" solution would be sub-optimal in case your DataTable can contain a lot of rows because it iterates twice over the row collection. This would be better, albeit a bit "uglier":

int highestID = -1;
int indexOfHighestID = -1;
for (int i=0; i<yourDataTable.Rows.Count; i++)
{
    int id = row.Field<int>("ID");
    if (id > highestID)
    {
        highestID = id;
        indexOfHighestID = i; 
    }
}
if (indexOfHighestID > 0)
{
    DataRow rowWithHighestID = yourDataTable.Rows[indexOfHighestID];
    // do something with that row here
}

Another Edit:

If you need to do that regularly in your application, the best way would be to define a primary key on the ID-column and somewhere keep the highest/last ID you inserted around so that you don't have to find out which one it was. Then it would be simply this:

DataRow rowWithHighestID = yourDataTable.Rows.Find(highestID);