New method handleMultiFileUpload and a helper method fileUploadHelper to hold identical code of handleFileUpload and handleMultiFileUpload.

handleMultiFileUpload allows the upload of multiple files at once.
It is additional code that should not break anything for existing portals.

The matching VHIRL-33 commit on VHIRL-portal shows the working client side changes needed for multifileuload.
Merge pull request #8 from alexkrumpholz/feature/VHIRL-33_multifile_upload

New method handleMultiFileUpload and a helper method fileUploadHelper to...