Does the warning give any specifics? Microsoft notes that:
SmartScreen Filter checks files that you download from the web against a list of reported malicious software sites and programs known to be unsafe. If it finds a match, SmartScreen Filter will warn you that the download has been blocked for your safety. SmartScreen Filter also checks the files that you download against a list of files that are well known and downloaded by many Internet Explorer users. If the file that you’re downloading isn’t on that list, SmartScreen Filter will warn you.
and
When you download a program from the Internet, SmartScreen Filter will check the program against a list of programs that are downloaded by a significant number of other Internet Explorer users and a list of programs that are known to be unsafe. If the program you’re downloading isn’t on either list, SmartScreen Filter will display a warning that the file isn’t “commonly downloaded.” It doesn’t necessarily mean the website is fraudulent or that the program is malware, but you probably shouldn’t download or install the program unless you trust the website and the publisher.
The plugin is a Windows zipfile that has been renamed from “spirix_shapefile_importer.zip” to “spirix_shapefile_importer.rbz” … the only thing inside the archive is “spirix_shapefile_importer.rb” which is a Ruby text file that you can read. You can either instruct SmartScreen to go ahead and let you proceed, or you can try the unzipped version here:
https://sites.google.com/site/spirixcode/code/spirix_shapefile_importer.rb