Use new format in determining destination.

When figuring out what the filepath should be for the converted file,
we temporarily change the item format to represent the format of the
converted file.  After determining what the path is, we restore the
format attribute so the rest of the codepath logic checks out.

beetsplug/convert.py

@@ -355,8 +355,12 @@ class ConvertPlugin(BeetsPlugin):
         item, original, converted = None, None, None
         while True:
             item = yield (item, original, converted)
+            # item format should represent the converted format, not the original
+            original_format, new_format = item.format, fmt.upper()
+            item.format = new_format
             dest = item.destination(basedir=dest_dir, path_formats=path_formats)
-
+            # need to restore this attribute for checks further along
+            item.format = original_format
             # Ensure that desired item is readable before processing it. Needed
             # to avoid any side-effect of the conversion (linking, keep_new,
             # refresh) if we already know that it will fail.

docs/changelog.rst

@@ -80,6 +80,8 @@ Bug fixes:
 
 - |BeetsPlugin|: load the last plugin class defined in the plugin namespace.
   :bug:`6093`
+- Fixed convert plugin not taking into account the new format when determining
+  the target path. :bug:`1360`
 
 For packagers: