Some email clients (particularly Gmail) will incorrectly add an extension to the license file, even though it already has one.
To fix this, select the file in the Finder, right-click it, and select Get Info. Under the "Name & Extension" box, make sure the extension is of the format '.applicationlicense' and doesn't end with '.xml' or something similar. If it does, remove the additional extension.
If this doesn't correct the issue, Contact Us and we'll work things out.