DrDooom:
otherwise think of Object "File" instead of "String".
Would not make any sense, none at all (File + File = Disk?) 
If you want your sketch to be "long term stable", you should reserve whatever memory, objects and what else you need in the setup phase of your sketch. The memory management (or lack thereof) on the Arduino is as crude as it gets, and the usage of dynamic memory during runtime (allocating, deallocating, reallocating) will cause memory fragmentation which may result in lockup or malfunction