Summary

Enables structured image reading for internal Base64BinaryFile files in FileSystem.read_file_structured() so vision models receive image payloads when reading agent-created images, adds a clear error guard in FileSystem.replace_file_str() to prevent invalid text replacements on binary files, and extends MIME detection in read_state_images prompt generation for GIF and WebP image files.

Problem

1. When binary/image files (PngFile, GifFile, JpgFile, JpegFile, WebpFile) are written to the agent's file system, calling read_file_structured() treated them as text. It extracted file_obj.read() (the summary string [binary <ext> file, <N> bytes]) and returned images=None. Consequently, invoking the read_file action on internal images gave the vision model no image data in ActionResult.images.
2. Calling replace_file_str() on binary files attempted string replacement on the stub description and passed it back to file_obj.write(), resulting in a base64 decode validation failure instead of a clear error.
3. In AgentMessagePrompt, read_state_images only explicitly identified .png and defaulted all other image files to image/jpeg, mislabeling .gif and .webp payloads.

Solution

1. In FileSystem.read_file_structured(): check isinstance(file_obj, Base64BinaryFile). If true, populate result['images'] = [{'name': file_obj.full_name, 'data': file_obj.content}] and set result['message'] = f'{sanitize_note}Read image file {resolved}.', matching external image read behavior.
2. In FileSystem.replace_file_str(): guard against isinstance(file_obj, Base64BinaryFile) and return Error: Cannot replace text in binary file '{full_filename}'. Overwrite it instead., matching append_file_content.
3. In browser_use/agent/prompts.py: extend MIME detection in AgentMessagePrompt for .gif (image/gif) and .webp (image/webp) so the model receives the correct media type.

Testing

• Added test_read_internal_binary_image_returns_images_payload verifying read_file_structured and read_file return the image data payload for internal binary files.
• Added test_replace_file_str_on_binary_file_returns_clear_error verifying replace_file_str returns a descriptive error and does not corrupt existing binary files.
• Added tests in test_agent_message_prompt_images_included verifying GIF and WebP MIME types and data URLs in read_state_images.
• Ran pytest on all related test suites (tests/ci/test_file_system_images.py, tests/ci/test_file_system_llm_integration.py, and tests/ci/infrastructure/test_filesystem.py): all 124 tests passed.
• Ran ruff check, codespell, and pyright: 0 errors.

Related Issue

Closes #6115