Add new fallback image for previews
Add doc preview adapter (shows just a doc.png)
Add docx preview adapter (shows just a docx.png)
Add ppt preview adapter (shows just a ppt.png)
Add pptx preview adapter (shows just a pptx.png)
Add xls preview adapter (shows just a xls.png)
Add xlsx preview adapter (shows just a xlsx.png)
Add pdf preview adapter (shows just a pdf.png)
Add zip preview adapter (shows just a zip.png)
Add txt preview adapter (shows just a txt.png)
Merge pull request #26 from 4teamwork/mle-26-better-default-images

New default images for several contenttypes