bug#39117: GNOME Files: No file thumbnails

[sirgazil via Bug reports for GNU Guix <bug-guix@gnu.org>]

GNOME Files (nautilus) does not display thumbnails of files like images, videos, and other files that usually have thumbnails. This makes it hard to browse large directories of multimedia files. 


## Steps to reproduce

1. Start GNOME Files.
2. Make sure that  "GNOME Files → Preferences → Search & Preview → Thumbnails" is active ("Files on this computer" is the option selected in my case for files less than 10 MB).
3. Browse to a directory that contains images.


## Unexpected result

All images are represented by the same icon, so you can only differentiate them by file name (when the files are not generated by devices like cameras).

Screenshot: https://multimedialib.files.wordpress.com/2020/01/guix-gnome-files-3.32.3-no-image-previews-2020-01-13.png


## Expected result

You can see thumbnails of all images so you can browse them by their content, not only by their file name.


## System information

GNOME 3.32.2
GNOME Files 3.32.3-stable

$ LANG=C guix describe
Generation 8	Jan 12 2020 16:24:29	(current)
  guix f9ea47a
    repository URL: https://git.savannah.gnu.org/git/guix.git
    branch: master
    commit: f9ea47a978882541063dd0d77c138be881ce0688


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