Cannot understand why this happening.
On fresh installation of 5.2 jar-packed module file placed in (root)/modules/platform is loaded upon startup, but for 5.1 it does not event see it.
AFAIK 5.1 does not use the "modules" directory at all, though I must admit I never use the installer and can't speak for every individual release.
Thanks for reply.
I tested this on fresh install of Community 201605 (witch is also 5.1) - and it worked.
today tested on frsh installation of 201602 community version - it worked.
so, maybe some other module disables loading extensions from (root)/modules?
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