The autor himself provide an alternate procedure.
As you can see in their product description www.nonumber.nl/betterpreview?tab=Download
Their installation is still experimental and may have problems.
This is the reason why they provide a manual procedure that you can find in the INSTALL.txt of their product.
I have found another workarround
procedure to allow JMS know that the extension exists..
This one has been tested with BetterPreview version 1.3.0
You have to unzip
the extension in the directory /administrator/components/com_nonumber-installer-UNINSTALLME
Becarefull that on Unix platform, the directory name is case sensitive.
When this is done, you can install the extension.
Remark: The com_nonumber-installer-UNINSTALLME directory will be automatically removed by the BetterPreview installation procedure .