Prefix office import artifact names with the name of the imported office file

The office import process can generate artifacts (e.g. images found in the office file) which are attached to the target document. Currently the names of these artifacts don't depend on the name of the imported office file (e.g. output_html_d45e2.jpg or presentation.zip). This can cause name conflicts and makes it hard to detect the source of an artifact when multiple office files are imported in the same target document.