I forgot already why windows build is without having MagicMime support added, and failed to find a reason in the archives.
But there is demand from Windows users! 
Without such functionality we cannot consistently (cross-platform) use git-annex for many DataLad datasets, where text files (well -- soon "non-binary files") are configured to be committed directly to git.
It's a C library and it's not vendored into the haskell library as is often the case, so it has be be linked in. I don't know what would be involved in getting it installed on windows and bundled with git-annex in a usable state, and I'm a bit afraid to find out.
WSL can be used to run linux git-annex on windows, so that's a way already available to use MagicMime.
(The entire future of the windows port is currently TBD, it seems likely that it will go away and WSL be used, aside from the current bugs in WSL that make sqlite crash.)
FWIW:
I am not yet sure if WSL would ever become a panacea. So far in my attempts I kept running into issues which related to how you (user) expose Windows file system(s) inside WSL, and there might be no "one setup which fits all the use cases".
There seems to be efforts like https://github.com/nscaife/file-windows which seems to promise building Windows binaries of file and libmagic for Windows - may be it could be used here.
IMHO an ideal way for git-annex deployment on windows would have been a conda-forge build of it. Unfortunately it would not facilitate solution here since I do not see windows build for libmagic. (meanwhile filed an issue, if resolved - could give an additional motivation to approach git-annex-feedstock/issues/15
I guess that
git annex adjust --unlockwould avoid whatever the problem is with the symlinks.fileusing/as shown in https://github.com/nscaife/file-windows/pull/9file. releases provide builds using difference VS versions, not sure if it needs any VS runtimem2-filepackage (a bit dated from 3 years ago)so may be one of those ways could assist in establishing a built with MAGIC support on windows?
FWIW, the mission of automated building of windows installer for annex with libmagic support is accomplished.