file_packager.py: Use last occurrence of @ in --preload-file

This means that `@@` escaping is optional for the targets rather
than the source.

Fixes #15947

ChangeLog.md

diff --git a/ChangeLog.md b/ChangeLog.md
index 68c4a0a..8db237a 100644
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -23,6 +23,9 @@
 - `wasmX` test suites that are defined in `test_core.py` have been renamed to
   `coreX` to better reflect where they are defined.  The old suite names such
   as `wasm2` will continue to work for now as aliases.
+- When using `--preload-file` the last `@` is not taken as the dividing point
+  between the source and target filenames.  As previously, `@` can be included
+  explicitly by escaping using `@@`.
 
 3.1.1 - 08/01/2022
 ------------------

tools/file_packager.py

diff --git a/tools/file_packager.py b/tools/file_packager.py
index da56bed..bce114e 100755
--- a/tools/file_packager.py
+++ b/tools/file_packager.py
@@ -237,7 +237,7 @@
       mode = leading
       # position of @ if we're doing 'src@dst'. '__' is used to keep the index
       # same with the original if they escaped with '@@'.
-      at_position = arg.replace('@@', '__').find('@')
+      at_position = arg.replace('@@', '__').rfind('@')
       # '@@' in input string means there is an actual @ character, a single '@'
       # means the 'src@dst' notation.
       uses_at_notation = (at_position != -1)