Do not allow to drop several files to nuxeo-dropzone with multiple set to false

Steps to reproduce:

1. Log in as Administrator and navigate to the personal workspace
2. Click on the + button and choose File
3. With your File explorer, select multiple files on your disk and drop them to the Content field
4. Observe that all the files are correctly uploaded and displayed in the Content dropzone whereas only one file should be allowed

Expected behavior: when uploading multiple files to the nuxeo-dropzone element (with multiple=false, which is the default value), there should either be an error (indicating that only one file can be uploaded in this dropzone), or only one (the first one?) file should be successfully uploaded and the other are rejected.