string test = "JOBNO_2014-01-0127-CON_154156_URI-Supply-BLANK-77.xlsm";
int underscoreIndex1 = test.IndexOf('_');
int underscoreIndex2 = test.IndexOf('_', underscoreIndex1+1 );
string result = test.Substring(underscoreIndex1+1, underscoreIndex2 - underscoreIndex1-1);
System.Diagnostics.Debug.WriteLine(result);