knotserver/xrpc: set Content-Type when serving images

svgs were broken since they defaulted to text/plain instead of
image/svg+xml.

Signed-off-by: Anirudh Oppiliappan <[e-mail]>

knotserver/xrpc/repo_blob.go

···
 				return
 			}
 			w.Header().Set("ETag", eTag)
+			w.Header().Set("Content-Type", mimeType)
 
 		case strings.HasPrefix(mimeType, "text/"):
 			w.Header().Set("Cache-Control", "public, no-cache")