Only set mode when necessary

src/PIL/FtexImagePlugin.py

@@ -79,8 +79,6 @@ def _open(self) -> None:
         self._size = struct.unpack("<2i", self.fp.read(8))
         mipmap_count, format_count = struct.unpack("<2i", self.fp.read(8))
 
-        self._mode = "RGB"
-
         # Only support single-format files.
         # I don't know of any multi-format file.
         assert format_count == 1
@@ -95,6 +93,7 @@ def _open(self) -> None:
             self._mode = "RGBA"
             self.tile = [ImageFile._Tile("bcn", (0, 0) + self.size, 0, (1,))]
         elif format == Format.UNCOMPRESSED:
+            self._mode = "RGB"
             self.tile = [ImageFile._Tile("raw", (0, 0) + self.size, 0, "RGB")]
         else:
             msg = f"Invalid texture compression format: {repr(format)}"