Confirming this problem for Joomla! 2.5 and Hotspots Core 3.5.4:
https://compojoom.com/forum/52-newbie-area/18021-problems-with-hotspots-moving-to-joomla-3#21699
Details:
When uploading, the file permissions are 600 instead of 644, so that on some server configurations the file isn't readable from the webserver (giving 403-Forbidden instead). As a workaround, I need to edit these file permissions manually via FTP.