What kind of attachment? Word, Excel, PowerPoint, pdf? For the first two in particular, look at the huge number of filetypes they support. Is it reasonable to expect a mail client to have parsers for all these types of files?

If you can't make sense of the content of files then you can't usefully search them. :-( And note that as they are stored in email messages, they will be encoded so that their content can be represented as printable characters, to meet some fundamental limitations of email, and so don't actually contain the explicit text that you see in the attachments when you do open them. This means that an external search tool looking at your stored mail is unlikely to be able to make sense of the content of attachments.

If you regularly need to search attachments then you need to set up something such a a message filter in Thunderbird to save copies of the attachments as regular files, and then you can use whatever tools you normally use for searching the kinds of files you work with. The Filtaquilla addon provides useful enhancements to Thunderbird's filters.

can you tell me which add-on are compatible with attachment content search.

Kamaljeetyadav said

can you tell me which add-on are compatible with attachment content search.

save the attachments to your hard disk and ask windows to search them for you. It will do a better job than Thunderbird can. Still can't search them? Then they can not be searched.