URL alias for File field on a Content Type
Well attachment links module does exactly what you want, this module is still being maintained, the fact is that there is not a new stable release for this module, but the manteiners are solving issues in the issues queue, and they commit changes to the 7.x-1.x-dev download that version and you will see it works like a charm. Check also Upload replace module is good also use the 7.x-1.x-dev version of this module. Both modules are for the same purpose but they do it in a different way.