Extension:UploadWizard/Error behavior

Invalid extension:
 * Report on first step and reject this file (but accept other valid files)

Too large:
 * Report on first step and reject this file (but accept other valid files)

Duplicate file contents:
 * Report on first step (immediately after upload) and reject this file (but accept other valid files)

File was previously deleted:
 * Minimally acceptable behavior: Report on first step (immediately after upload) and reject this file (but accept other valid files)
 * Ideal: As above, but trigger an interactive warning that allows the user to override.

File with same name was previously deleted:
 * Discard and ignore this warning (this is pretty useless information in almost all cases)

File name checks:
 * File name matches title blacklist,
 * File name exists
 * File name exists once name is normalized (e.g. "JPEG" to "jpg")
 * File name is protected
 * File name includes invalid characters
 * Because the file is initially uploaded to the temporary stash using a unique identifier, all these checks only need to be performed at the third stage ("Describe"), at which point the user should be forced to choose valid titles for all files
 * If for whatever reason a bad name slips through the third step, there is a final catch-all in the fourth step allowing the user to correct file names.