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In general, you can use any Lucene query, with the following fields available:
- content, which searches inside attachment file contents. It's the default field if you don't specify any field explicitly.
- name, which searches by names of the attachment files excluding file extensions;
- type, which searches by file extensions (pdf, doc, txt, xls, odt, ods, xml, html, ...)
Advanced syntax
Syntax used for writing queries is Lucene syntax. More details how to write advanced queries can be found in Lucene documentation.