It depends largely on the file type. You could split it as long as it's a raw format and as long as you place the correct header data in the new file. If it is compressed, it becomes much more difficult, but still do-able. I agree with Totte. It would be better time spent to find a program on tucows or download.com unless this is something that your program will be doing on a regular basis, i.e. it's a feature of the program.
Chris