Update file extensions in README.

Author: dbkr

README.md

@@ -84,7 +84,7 @@ logs.)
 * `file`: an arbitrary file to attach to the report. Saved as-is to disk, and
   a link is added to the github issue. The filename must be in the format
   `name.ext`, where `name` contains only alphanumerics, `-` or `_`, and `ext`
-  is one of `jpg`, `png`, `txt` or `json`.
+  is one of `jpg`, `png`, `txt`, `json`, `txt.gz` or `json.gz`.
 
   Not supported for the JSON upload encoding.