If you are searching for an attachment with text in a non-western language and the attachment is not included in the search results, the attachment might not be searchable.
Documents in non-western languages that do not have a character set (charset) parameter identified in the content type header of the attachment might not be indexed correctly. The character set parameter identifies the type of character encoding, which is how Zimbra identifies how to index the content. Attachments must be indexed correctly to be included in searches. This issue usually happens to files that are plain text attachments or csv attachments.