Clean-up some code in attachment::from_file
Clean-up some code in attachment::from_file

bkircher/ews-cpp,otris/ews-cpp,m-Schlitzer/ews-cpp,otris/ews-cpp,m-Schlitzer/ews-cpp,otris/ews-cpp,bkircher/ews-cpp,otris/ews-cpp,m-Schlitzer/ews-cpp,bkircher/ews-cpp