code: protected boolean isAvailable() {
          // If in memory, it's available.
          if (fileItem.isInMemory()) {
              return true;
          }
          // Check actual existence of temporary file.
          if (fileItem instanceof DiskFileItem) {
              return ((DiskFileItem)fileItem).getStoreLocation().exists();
          }
          // Check whether current file size is different than original one.
          return (fileItem.getSize() == size);
      }
docs: Determine whether the multipart content is still available. If a temporary file has been moved, the content is no
  longer available.