Redesign dropzone file previews

Previous design was not very functional:

• Did not properly support image thunbnails.
• Fixed width makes the filename to get truncated
• A permanent grey sqare that serves no purpose

Redefine some markup and options to improve some of those:

• Set scaling as method for image thumbnail, instead of cropping.
• Fix image rendering for thumbnails.
• Allow flexible width and height of the preview container. This way
  filenames are fully shown.