When you save file in database it normally save in Byte(binary) format, you can save it back to file using File.WriteAllBytes() method, it Creates a new file, writes the specified byte array to the file, and then closes the file. If the target file already exists, it is overwritten.
see below snippet
Byte objByte = (Byte)dr["col1"];
In above example I have take file format as JPG cause I know the file which I have saved in database is .JPG, you should know the file extension/format before retrieve frile from database