Fix nuxeo-document-import creating Documents using removed files

After importing a set of 6-10+ files using nuxeo-document-import and quickly removing all but one of the files in succession, Documents are created using removed files.

Steps to Reproduce:

1. In Web UI, select and drop a set of 6-10+ files into the dropzone of nuxeo-document-import.
2. Immediately after, click the "X" button of the top-rightmost file repeatedly to clear all but one of the imported files.
3. Once one file is remaining, click the Create button.

Expected behavior: only one Document is created from the remaining file.

Actual behavior: several Documents are created using both the remaining file and an arbitrary set of files removed from the set.

(see the attached video for reference)