Not currently possible, and attachments within messages are not indexed for searching.
However, with Mail frontmost, check the Scripts menu. DEVONthink Pro Office includes a script that will separately add a selected message and its attachments to DEVONthink. Attachments added in this way will be indexed for searching.
You might need to activate DEVONthink > Install Add Ons… > Script Menu Extra > Applications Scripts to insure those scripts are installed for Mail’s scripts menu.