Greenhopper 5.10.5.1 download is an invalid jar

When attempting to install GreenHopper 5.10.5.1 in an installation that was just upgraded in-place from Jira 5.0.3 and GH 5.10.5, I received the error message "Could not find a handler capable of installing the plugin at jira-greenhopper-plugin-5.10.5.1.jar. Check that the file is a valid plugin."
THe following error appeared in the atlassian-jira.log file:
2012-07-25 08:45:34,328 pool-14-thread-3 ERROR (username) 525x762x1 1r4a4pl 172.28.161.123,172.28.163.30 /rest/plugins/1.0/ [com.atlassian.upm.PluginInstaller] File 'jira-greenhopper-plugin-5.10.5.1.jar' is not a valid plugin: Could not open the file as a jar

After that, I downloaded the file again and tried the following using Java 6 on my PC:
C:\Documents and Settings\username\My Documents\download\atlassian>jar -tvf jira-greenhopper-plugin-5.10.5.1.jar
java.util.zip.ZipException: error in opening zip file
at java.util.zip.ZipFile.open(Native Method)
at java.util.zip.ZipFile.<init>(ZipFile.java:127)
at java.util.zip.ZipFile.<init>(ZipFile.java:88)
at sun.tools.jar.Main.list(Main.java:979)
at sun.tools.jar.Main.run(Main.java:224)
at sun.tools.jar.Main.main(Main.java:1149)

C:\Documents and Settings\username\My Documents\download\atlassian>md5sum jira-greenhopper-plugin-5.10.5.1.jar
d6eca771bef25552349f189ae7eb7821 *jira-greenhopper-plugin-5.10.5.1.jar