Your code tries to change a binary buffer into a string. Strings are Unicode in NET so two bytes are required. The resulting is a bit unpredictable as you can see.
Just use a BinaryReader and its ReadBytes method
using(FileStream fs = new FileStream(@"C:\file", FileMode.Open, FileAccess.Read))
{
using (var reader = new BinaryReader(fs, new ASCIIEncoding()))
{
byte[] buffer = new byte[10];
buffer = reader.ReadBytes(10);
if(buffer[0] == 31 && buffer[1] == 139 && buffer[2] == 8)
// you have a signature match....
}
}