Support <input type="file" multiple>
Support <input type="file" multiple>

If several files are uploaded using a single input field (multiple), enumerate files and make them all available.

If 3 files are uploaded using:
<input name="upload" type="file" multiple>

They are named by ```session.parseBody(files);```:
upload
upload2
upload3

This way backward compatibilty is preserved.
updowndown/nanohttpd,anandsadu/nanohttpd,kayhanakbay/nanohttpd,EricCen/nanohttpd,ouya/nanohttpd,McFoggy/nanohttpd,gushedaoren/nanohttpd,weathersboyle/nanohttpd,vnnv/nanohttpd_vnnv,eighthave/nanohttpd,DavikChen/nanohttpd,WilliamDo/nanohttpd,vnnv/nanohttpd_vnnv,songfj/nanohttpd,biswajitind/SCMF,NanoHttpd/nanohttpd,songfj/nanohttpd,eighthave/nanohttpd,anandsadu/nanohttpd,guardianproject/nanohttpd,biswajitind/SCMF,McFoggy/nanohttpd,gushedaoren/nanohttpd,andreivisan/nanohttpd,LordFokas/nanohttpd,guardianproject/nanohttpd,andreivisan/nanohttpd,codebeautiful/nanohttpd,ma5069073/nanohttpd,weathersboyle/nanohttpd,panxw/nanohttpd,eGxC/nanohttpd,harshjv/nanohttpd,codebeautiful/nanohttpd,huanyi0723/nanohttpd,updowndown/nanohttpd,NanoHttpd/nanohttpd,LordFokas/nanohttpd,kayhanakbay/nanohttpd,huanyi0723/nanohttpd,QumuloMe/nanohttpd,ma5069073/nanohttpd,ouya/nanohttpd,panxw/nanohttpd,DavikChen/nanohttpd,harshjv/nanohttpd,QumuloMe/nanohttpd,eGxC/nanohttpd,WilliamDo/nanohttpd,EricCen/nanohttpd