Add filename extension to cache files

[concubidated]

This would implement FR #1476
I will let @shakesoda decide if to kill it or merge it.

[dougshell]

Is there any utilitarian use to this other than conforming to Windows file naming standards (having file extensions)

If not, this can still likely be merged if it does not cause complications elsewhere.

[concubidated]

only thing I can think of is the old cache would remain, and everything else would recache, not a big deal.

[dougshell]

the functional impact of that would be nil.

Is this really just a naming convention issue to adhere to Windows standard of having filename.fileextension

[Scraticus]

Would this change affect any of the caching/score/stat systems used in 5.1?

[concubidated]

This would cause the old cache to remain there, and for the system to recache everything. again. Personally I think it should just be closed, as its pretty silly. Nothing would change the score/stats system here