One of the reasons outlook needs to be "protected" from malware so much is features like the one you mention. Thunderbird does nothing with attachments, just stores them in the text based mime format they were sent across the internet. So to search the content of an attachment, it has to be decoded from mime, written to disk, opened and searched.and that also predicates that their is a search helper that understands the search file format format. It is in opening attachments that most email malware infects the computer. So Thunderbird does not do anything with the attachment, so getting an email is not an issue. Outlook does things with the attachment so tight anti virus scanning is required and even them regularly fails. So not only am I unaware of an addon that searches attachment content. I would not install one for security reasons.