-(NSString *) registerPlainText:(NSString *)filename
{
    if ([_dict valueForKey:filename] == nil) {
    NSError *error;
  
        NSString *path = [[NSBundle mainBundle] pathForResource:filename];
        NSString *content = [NSString stringWithContentsOfFile:path encoding:NSUTF8StringEncoding error:&error];
    
    if (error != nil) {
      NSLog(@"Error while loading plain text file: %@", error);
    }
        
        return (NSString *) [self registerAsset:filename withContent:content];
    } else {
        return (NSString *) [self registerAsset:filename withContent:nil];
    }
}