Fix preview update after blob update

Steps to reproduce:

1. Create a File document with no attachment
2. Drag'n drop a .docx file to the D&D area
3. Observe that the preview is updated but also that the .docx file is downloaded

The problem sounds related to nuxeo-document-preview.js#_updatePreview which is called twice when the document is updated : the first time, it dowloads the previewer whose mime-type is ".*", the second time it downloads the PDF rendition as expected.

Expected behavior: the preview is updated once the file is uploaded with the D&D but no file download happens