Adds new file only if before_add was successful
Adds new file only if before_add was successful

With this change before_add can be used to do javascript validation.
After an invalid file was not uploaded the next valid upload will 
trigger 's3_uploads_complete' and not only 's3_upload_complete'
raycheung/s3_direct_upload,danieljacobarcher/s3_direct_upload,gertfj/s3_direct_upload,ncri/s3_direct_upload,Cadasta/s3_direct_upload,gertfj/s3_direct_upload,gertfj/s3_direct_upload,adveli/s3_direct_upload,maletor/s3_direct_upload,ilyakatz/s3_direct_upload,PixelPerfectTree/s3_direct_upload,truenito/s3_direct_upload,acnalesso/s3_direct_upload,tim-vandecasteele/s3_direct_upload,waynehoover/s3_direct_upload,luddement/s3_direct_upload,RobotsAndPencils/s3_direct_upload,prasann/s3_direct_upload