The downloads of jar files are served with a "text/plain" MIME type. This makes standard compliant browsers like Opera display the jar file instead of offering a download dialog.
Problem observed on: http://plugins.intellij.net/plugin/?id=256 when trying to download the latest version (7.14) by simply clicking the download link.
Please fix this.
P.S.: the information about "text/plain" was taken by monitoring HTTP traffic with Fiddler, so Opera just interprets what's served.