Minor bugs and feature request: Spaces in file path, explain the path to ImageMagick

[Xendiadyon]

Hi aschempp,

I found a small bug in your extension.
If our customer uses a space in the path to the pdf (yes, I know that this is bad habit) the Imagemagick will throw an extension.
You have to change ContentPreviewDownload.php, l. 163 (exec, sprintf) and add quotes:
'PATH=\$PATH:%s;export PATH;%s/convert %s\'%s/%s%s\' %s/%s 2>&1',

Furthermore, you should explain somewhere that the path to ImageMagick has to be set in the Contao Settings and how the path should look like:
https://community.contao.org/de/showthread.php?65801-PDF-Thumbnails-mit-quot-contao-previewdownload-quot&p=480257#post480257

I would like to suggest the possibility to have this functionality for a list of downloads as well.