Always return "text/plain" for commit message file type

Change-Id: Ic881059efa54cd24e6e2c1f211657adcb8338ad6

Documentation/rest-api-changes.txt

diff --git a/Documentation/rest-api-changes.txt b/Documentation/rest-api-changes.txt
index 6fa99c0..07cb86a 100644
--- a/Documentation/rest-api-changes.txt
+++ b/Documentation/rest-api-changes.txt
@@ -2826,7 +2826,8 @@
   GET /changes/myProject~master~I8473b95934b5732ac55d26311a706c9c2bde9940/revisions/674ac754f91e64a0efb8087e59a176484bd534d1/files/readme.txt/type HTTP/1.0
 ----
 
-The content MIME type is returned as string.
+The content MIME type is returned as string. The content type for the commit
+message (`/COMMIT_MSG`) is always returned as "text/plain".
 
 .Response
 ----