Earlier this evening I was working on a project and put together a quick batch file to uninstall and reinstall a solution.  I issued the following command (in a batch file):

STSADM -o addsolution -filename XmlWebParts.WSP

and I got back in response:

“xmlwebparts.wsp” has an unsupported extension, and cannot be added to the solution store.

Having never seen this particilar error from STSADM I was intrigued.  I decided to try the following command:

STSADM -o addsolution -filename XmlWebParts.wsp

and received:

Operation completed successfully.

Say what?  It turns out that for some reason STSADM -o addsolution wants the .wsp extension in lower case.