refactor: simplify image file processing to use provided filenames directly

aider/coders/base_coder.py

@@ -728,8 +728,10 @@ class Coder:
             return None
 
         image_messages = []
-        for fname, content in self.get_abs_fnames_content():
-            if is_image_file(fname):
+        for fname in fnames:
+            if not is_image_file(fname):
+                continue
+
             mime_type, _ = mimetypes.guess_type(fname)
             if not mime_type:
                 continue