Add css for the iframes for e2e attachments

src/skins/vector/css/matrix-react-sdk/views/messages/MImageBody.css

@@ -44,3 +44,17 @@ limitations under the License.
     vertical-align: middle;
     pointer-events: none;
 }
+
+/* Remove the border and padding for iframes for download links. */
+.mx_MImageBody_download iframe {
+    margin: 0px;
+    padding: 0px;
+    border: none;
+    width: 100%;
+    /* Set the height of the iframe to be 1 line of text.
+     * Iframes don't automatically size themselves to fit their content.
+     * So either we have to fix the height of the iframe using CSS or
+     * use javascript's cross-origin postMessage API to communicate how
+     * big the content of the iframe is. */
+    height: 1.5em;
+}