Fixed preview overlay showing by default

This was a regression made visible by the recent change to
enforce dimensions on this overlay, which in turn visualizes
the blocked/unblocked states of the preview (see fc6d6ffad)
Show label-less input#file field in UploadField with canUpload=false

The field carries the configuration, and some non-upload functionality
like "attach files" still relies on the fileupload jQuery plugin
being initialized.