If it is still nit working for you try below code it's working for me:

int image_buf_size = 4000 * 1000;
uint8_t *image = (uint8_t *)ps_calloc(image_buf_size, sizeof(char));

size_t length=fb->len;

size_t olen;

Serial.print("length is");

Serial.println(length); int err1= mbedtls_base64_encode(image, image_buf_size, &olen, fb->buf, length);

Serial.println(err1);

String img((const __FlashStringHelper*) image);

Serial.println(img); //msg1=img;

If it is still not working for you try below code it's working for me:

 int image_buf_size = 4000 * 1000;                                                  
 uint8_t *image = (uint8_t *)ps_calloc(image_buf_size, sizeof(char));
                                                                                     
 size_t length=fb->len;
                                                                                     
 size_t olen;
                                                                       
 Serial.print("length is");
                                                                    
 Serial.println(length);
 int err1= mbedtls_base64_encode(image, image_buf_size, &olen, fb->buf, length);
                                                                      
 Serial.println(err1);
                                                                                     
 String img((const __FlashStringHelper*) image);
                                                                       
 Serial.println(img);
 //msg1=img;