At present there are two common issues with downloading files from the Grasshopper Forum.

GHX

The XML based file format of GHX files sometimes causes your browser to see it as a "text" file to display as a webpage.

To get around this, right click the link and select "Save link as..." option from the context menu, then click save. On a Mac to "right click" you can hold down the control button on the keyboard simultaneously whilst clicking the mouse button.

The alternative is to use GH files which are the Binary file format for Grasshopper and since 0.8.0050 have been the default. GH files are much smaller than GHX files since they are compressed and they will not be modified by web-browsers, email-clients and virus-scanners. 

RAR

For some reason the Ning social network sites do not allow RAR files to be downloaded.

You will see a message like:

Access Denied

You don't have permission to access "https://storage.ning.com/topology/rest/1.0/file/get/2638548923?profile=original" on this server.

Reference #18.8d863554.1338285173.38fb61c1

The only way to get around this is to ask the original poster to replace it with a ZIP file.