You should try using your tool to import 1 new test and see what the URL to the attachment looks like and if you are able to access it within ALM 11.52. We found that new tests with attachments worked fine.
If that is the case, then you should be able to use my solution and continue to use your external tool for new tests.
However, if it doesn't work, please post to this forum because I am interested in knowing your solution.