Don't specify mimetype in ui-blob.c

But be sure to specify correct filename. This way, the client can hopefully
guess a sensible mimetype based on the filename suffix, and cgit can ignore
the issue altogether.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
diff --git a/ui-blob.c b/ui-blob.c
index 11589db..ab44659 100644
--- a/ui-blob.c
+++ b/ui-blob.c
@@ -36,7 +36,7 @@
 	}
 
 	buf[size] = '\0';
-	ctx.page.mimetype = "text/plain";
+	ctx.page.mimetype = NULL;
 	ctx.page.filename = path;
 	cgit_print_http_headers(&ctx);
 	write(htmlfd, buf, size);