correct color of pasted image meta text

Refs #212

Test Plan:
- Find an image on the web, right-click it, select "Copy Image"
- Go to a channel or dm in Slack, use <cmd>+v to paste the image and
  upload it
- Verify that the meta text above the resulting image displayed in
  channel is in the correct theme color

scss/modules/messaging/_attachments.scss

@@ -126,6 +126,12 @@
   }
 }
 
+.c-message_kit {
+  &__file__meta {
+    color: $base-font-color;
+  }
+}
+
 .attachment_group {
   &.has_container {
     background: $color-base;