How to handle encrypt and decrypt file in ASP.NET core web API

See more:

C#

encryption

API

What is the best way to implement encrypt and decrypt file content using asp.net core web API.
It should be compatible with swagger also. I have tried this with the same way as encrypt string but there is limitation of size length or may be incomplete file will send over API.
My requirement is file content should be encrypt at client end before API call via swagger or postman and should be decrypt at service end .
File content encrypt at client end then only data at transmission will safe.

What I have tried:

public static string encrypt(string PlainText, byte[] key, byte[] iv)
      {
          string sR = string.Empty;

          byte[] plainBytes = Encoding.UTF8.GetBytes(PlainText);

          GcmBlockCipher cipher = new GcmBlockCipher(new AesFastEngine());
          AeadParameters parameters =
                       new AeadParameters(new KeyParameter(key), 128, iv, null);

          cipher.Init(true, parameters);

          byte[] encryptedBytes = new byte[cipher.GetOutputSize(plainBytes.Length)];
          Int32 retLen = cipher.ProcessBytes
                         (plainBytes, 0, plainBytes.Length, encryptedBytes, 0);
          cipher.DoFinal(encryptedBytes, retLen);
          sR = Convert.ToBase64String(encryptedBytes, Base64FormattingOptions.None);


          return sR;
      }