import com.haulmont.cuba.core.global.*;

import java.util.UUID;

import com.haulmont.cuba.core.app.FileStorageService;

import com.haulmont.cuba.core.entity.FileDescriptor;

FileStorageService service = AppBeans.get(FileStorageService.NAME)

String str = "sdfgvsdfgvd";

FileDescriptor fileDesc = dataManager.load(FileDescriptor.class)

.id(UUID.fromString('2f390615-38e4-41b0-370b-e5933cfa22e3')).view("_local").optional().orElse(null);

try{

byte[] file = service.loadFile(fileDesc);

ByteArrayOutputStream fileOS = new ByteArrayOutputStream();

fileOS.write(file);

fileOS.write(str.getBytes());

service.removeFile(fileDesc);

service.saveFile(fileDesc, fileOS.toByteArray());

}catch(Exception e){

log.debug(e)

}