I realize this is too late as a (in my oppinion: less desirable) solution was already implemented in dev however I would like to urge the devs to strongly reconsider using any form of lists that needs to be manipulated with compatible classnames for each weapon.

There are basically 2 things that needs to be considered for attachment mounting, the first is mount type and the second is mod compatibility

Different weapons have different mount types such as legacy AKs will have a side-bracket system that only works with russian optics and most western guns (as well as arma3?) use 20mm picatinny rail and all the optics/attachments made for this are (mostly) interchangeable and finally there is for example the german claw mounts as seen on MP5 and other H&K weapons.

In my oppinion the best solution would be something akin to simply having an attribute in every attachment config what mount type its meant for, it could even be as easy as an integer identifier where for example 1 would be 20mm rail so then every red dot designed for 20mm would have attachment_type = 1 and would be compatible with every weapon that accepts this attachment_type.

One potential issue would be if two mods conflict on the identifier and use the same slot however the benefits far outweight this con by not only fixing the whole attachment issue alltogether but also introducing mod compatibility where one mod makes attachments for 20mm rails it will automatically fit for a completely other mod whose weapons accepts 20mm rail attachments with no further interaction or knowledge about the two different mods.

If I understood the current implementation right it would still mean that any new attachments introduced in a new addon would still have to modify all the possible weapons that should be able fit the new attachments which is a stupid solution and works against mod compatibility and it will just create a huge mess worse than what we saw with magazines in arma2