Excel2007: When using disk caching, do not read all the files into memory.

The potential gain in memory usage offered by disk caching was
annihilated by the fact that the generated content was read into memory
before being added to the archive.
Excel2007: do not uselessly read image data when zipping.

When we have a file name for the image being added to the ZIP file,
there is no need to read it into memory.