C++ file transfer service

We have a file transfer service. The backend is written in c++ the front end is written in c#. I need someone with experience with c and c++ the code is very old and I do not want to change it I just need it fixed. It loads files of all types to a stream. It has a but in in it and I do not have time to track it down. I need to make sure it works on all files and all file types. Thne important part is experience in c, c++ and c#.