Attempt memory mapping when tile args is a string

src/PIL/ImageFile.py

@@ -187,6 +187,8 @@ class ImageFile(Image.Image):
         if use_mmap:
             # try memory mapping
             decoder_name, extents, offset, args = self.tile[0]
+            if isinstance(args, str):
+                args = (args, 0, 1)
             if (
                 decoder_name == "raw"
                 and len(args) >= 3

src/PIL/WebPImagePlugin.py

@@ -168,6 +168,9 @@ class WebPImageFile(ImageFile.ImageFile):
 
         return super().load()
 
+    def load_seek(self, pos):
+        pass
+
     def tell(self):
         if not _webp.HAVE_WEBPANIM:
             return super().tell()