I did some checking and yes it seems you are correct. Albeit, not sure why this issue hasn't been reported until now. I have logged a bug to have it fixed in our next major release.
In the meantime, if you download the plugin jar file manually from Marketplace, you can open the plugin jar with a file compression utility like 7zip(winzip and others will probably work too) and edit the file name. Then install the plugin by clicking on the "Upload Plugin" link on the Confluence plugin manager page and selecting the file you just modified.
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