Problem

When binary/image files (PngFile, GifFile, JpgFile, JpegFile, WebpFile via Base64BinaryFile) are created in the agent's file system (e.g. via write_file tool):

1. FileSystem.read_file_structured() does not return the image data in result['images']. Instead, it treats the binary file as text, extracting file_obj.read() (the summary stub [binary <ext> file, <N> bytes]) and returning {'message': ..., 'images': None}. Consequently, when the agent invokes read_file on an internal image, the vision model receives no image payload in ActionResult.images.
2. FileSystem.replace_file_str() does not guard against Base64BinaryFile. It attempts string replacement on the stub description and passes it to file_obj.write(), which fails with a base64 validation error rather than cleanly explaining that binary files cannot have text replaced.

Current Behavior

• Calling await fs.read_file_structured('test.png') on an internal PNG file returns {'message': 'Read from file test.png.\n<content>\n[binary png file, 68 bytes]\n</content>', 'images': None}.
• Calling await fs.replace_file_str('test.png', 'old', 'new') on an internal binary file attempts to validate the description string as base64 and raises/returns: Error: content for 'test.png' is not valid base64....

Expected Behavior

• Calling await fs.read_file_structured('test.png') on an internal image file should populate result['images'] = [{'name': file_obj.full_name, 'data': file_obj.content}] and set result['message'] = 'Read image file test.png.', matching external image reads so read_file supplies images to the vision model.
• Calling await fs.replace_file_str('test.png', 'old', 'new') should cleanly return Error: Cannot replace text in binary file 'test.png'. Overwrite it instead., matching Base64BinaryFile.append_file_content.

Reproduction

import asyncio
from browser_use.filesystem.file_system import FileSystem

async def main():
    fs = FileSystem('test_workspace')
    png_1x1 = 'iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAC0lEQVR4nGNgAAIAAAUAAXpeqz8AAAAASUVORK5CYII='
    await fs.write_file('test.png', png_1x1)

    # 1. read_file_structured returns images=None
    res = await fs.read_file_structured('test.png')
    assert res['images'] is not None, f"Expected images list, got {res['images']}"

    # 2. replace_file_str crashes with base64 decode error
    replace_res = await fs.replace_file_str('test.png', 'binary', 'text')
    print(replace_res)

asyncio.run(main())

Technical Context

• Base64BinaryFile was introduced in PR filesystem: write_file can create tiny binary images (png/gif/jpg/webp) for upload flows #5280 to allow write_file to create real image files for upload workflows.
• browser_use/filesystem/file_system.py:
  • In read_file_structured(), lines 844-868 currently state # For internal files, only non-image types are supported and call file_obj.read(). For Base64BinaryFile, content is base64 string, so read_file_structured() should set result['images'] = [{'name': file_obj.full_name, 'data': file_obj.content}].
  • In replace_file_str(), lines 949-960 perform string replacement and rewrite the file without checking isinstance(file_obj, Base64BinaryFile).