As per your code, you are trying to search the file names in the given directory .,
for this you can use the below code.
string[] files = Directory.GetFiles(Server.MapPath("files/"), "*.*");
Second Argument can be any wild card.
Ex :
string[] files = Directory.GetFiles(Server.MapPath("files/"), "A*.*");
files start with "A"
string[] files = Directory.GetFiles(Server.MapPath("files/"), "ABC?.*");
Files Start with "ABC" and ends with any character.
string[] files = Directory.GetFiles(Server.MapPath("files/"), "ABC DEF*.*");
Files Start with "ABC DEF" and ends with any characters.
Hope this helps you.