This sadly doesn't work anymore when running Unleashed 200.7.10.102.64
The uploaded software version is 104.0.0.0.1347, doesn't match with Master AP's 200.7.10.102.64, please choose the same version as Master AP.
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There is an upgrade option per AP, when you upload the file in that field, it seems to work, under Select image file(s) & Upgrade , not the Preload Image feature.