In Documents, thumbnail images are created for the following file types:

  • bmp
  • doc
  • docx
  • gif
  • jpg/jpeg
  • pdf
  • odp
  • ods
  • odt
  • png
  • ppt
  • pptx
  • tiff/tif
  • xls
  • xlsx

If your file is not one of these types, then a generic thumbnail image is used.