handleRaw only streamed the blob behind an isImageMime gate (#221), so
non-image attachment pastes fell through to empty row.Content and /raw
served 0 bytes. Serve the blob for every attachment mime, passing the
sniffed mime through serveContentType so active-content types (html,
svg, xml) still serve as text/plain per the #34 rule. Regression tests
cover text and html attachments (size, Content-Type, byte equality).