Display an error/warning when uploading a non accepted blob

Steps to reproduce:

1. start nuxeo
2. edit nxserver/nuxeo.war/ui/document/file/nuxeo-file-create-layout.html
3. after line 48, add this to the nuxeo-dropzone element

           accept=".pdf,.docx"

4. save the file
5. log in to Nuxeo and do a hard refresh in the browser to be sure all the resources have been reloaded
6. navigate to a workspace, push the + button and choose File
7. in the File creation form, try to upload a XLS file
8. observe that the file is not uploaded and no message is displayed
9. only a warn is logged to the console, as expected by the code

This improvement request is to display a message in the creation form (or in the page with a toast message) to indicate that the uploaded file was not accepted