Hi,
According to smart search results webpart documentation, the
Attachment search settings are described as:
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Search in attachments: If it's true system also searches in document's attachments. SQL search indexing must be enabled and set correctly.
...so this makes the search to include result from indexed attachments. More details are listed in Smart search module documentation in DevGuide, article
Searching attachments. It links to another article about how to configure fulltext search in files over your DB.
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WHERE condition: Where condition for searching in document's attachments.
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ORDER BY expression: Order by expression for searching in documents's attachments.
Hope this helps.
Regards,
Zdenek