compose: Have at least 1 new line before and after an attachment.

Until now, an attachment was inserted in an inline manner, but now we
insert it as a block, with 0 blank lines before and after it.

web/src/upload.js

@@ -183,6 +183,8 @@ export async function upload_files(uppy, config, files) {
             compose_ui.insert_syntax_and_focus(
                 get_translated_status(file),
                 get_item("textarea", config),
+                "block",
+                1,
             );
             compose_ui.autosize_textarea(get_item("textarea", config));
             uppy.addFile({