As the comment states, it would be nice to load the Virus Scanner on
the Attachment (rather than the blob).
However, in order not to clobber the blob metadata, we want to run the
VirusScanner once the blob analyzer did run.
And the most direct way to detect that the blob analyzer did run is to
add an `on_update_commit` hook on the blob, as this hook will be
trigerred when saving changes to the metadata. This is what the current
solution uses.
So the current solution is almost optimal, and has a low chance of
accidentally clobbering the blob metadata – as the virus scanner is only
started when the analysis phase is finished.
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