Crashes when copying response to clipboard

Problem Description

I'm trying to copy a response to the clipboard, it crashes, and it asks me to file a bug here.

Steps to reproduce the behavior:

1. Go to response (8.5kb multipart/mixed response)
2. Type :, then export.clip raw_response @focus, then enter
3. Following is displayed:

  File "/nix/store/bavhjp7n9rkjzvzsavcw2zzyi9kh6xwf-python3.8-mitmproxy-5.2/lib/python3.8/site-packages/mitmproxy/master.py", line 86, in run_loop
    loop()
  File "/nix/store/cnidikkp18zrcrzh55ags3j18c0dw07y-python3.8-urwid-2.1.1/lib/python3.8/site-packages/urwid/main_loop.py", line 287, in run
    self._run()
  File "/nix/store/cnidikkp18zrcrzh55ags3j18c0dw07y-python3.8-urwid-2.1.1/lib/python3.8/site-packages/urwid/main_loop.py", line 385, in _run
    self.event_loop.run()
  File "/nix/store/cnidikkp18zrcrzh55ags3j18c0dw07y-python3.8-urwid-2.1.1/lib/python3.8/site-packages/urwid/main_loop.py", line 1494, in run
    reraise(*exc_info)
  File "/nix/store/cnidikkp18zrcrzh55ags3j18c0dw07y-python3.8-urwid-2.1.1/lib/python3.8/site-packages/urwid/compat.py", line 58, in reraise
    raise value
  File "/nix/store/np0b7p04kbhl244yv65d2sc37kk7wcqf-python3-3.8.5/lib/python3.8/asyncio/events.py", line 81, in _run
    self._context.run(self._callback, *self._args)
  File "/nix/store/cnidikkp18zrcrzh55ags3j18c0dw07y-python3.8-urwid-2.1.1/lib/python3.8/site-packages/urwid/raw_display.py", line 416, in <lambda>
    wrapper = lambda: self.parse_input(
  File "/nix/store/cnidikkp18zrcrzh55ags3j18c0dw07y-python3.8-urwid-2.1.1/lib/python3.8/site-packages/urwid/raw_display.py", line 515, in parse_input
    callback(processed, processed_codes)
  File "/nix/store/cnidikkp18zrcrzh55ags3j18c0dw07y-python3.8-urwid-2.1.1/lib/python3.8/site-packages/urwid/main_loop.py", line 412, in _update
    self.process_input(keys)
  File "/nix/store/cnidikkp18zrcrzh55ags3j18c0dw07y-python3.8-urwid-2.1.1/lib/python3.8/site-packages/urwid/main_loop.py", line 513, in process_input
    k = self._topmost_widget.keypress(self.screen_size, k)
  File "/nix/store/bavhjp7n9rkjzvzsavcw2zzyi9kh6xwf-python3.8-mitmproxy-5.2/lib/python3.8/site-packages/mitmproxy/tools/console/window.py", line 310, in keypress
    k = super().keypress(size, k)
  File "/nix/store/cnidikkp18zrcrzh55ags3j18c0dw07y-python3.8-urwid-2.1.1/lib/python3.8/site-packages/urwid/container.py", line 1123, in keypress
    return self.footer.keypress((maxcol,),key)
  File "/nix/store/bavhjp7n9rkjzvzsavcw2zzyi9kh6xwf-python3.8-mitmproxy-5.2/lib/python3.8/site-packages/mitmproxy/tools/console/statusbar.py", line 201, in keypress
    return self.ab.keypress(*args, **kwargs)
  File "/nix/store/bavhjp7n9rkjzvzsavcw2zzyi9kh6xwf-python3.8-mitmproxy-5.2/lib/python3.8/site-packages/mitmproxy/tools/console/statusbar.py", line 149, in keypress
    self.prompt_execute(text)
  File "/nix/store/bavhjp7n9rkjzvzsavcw2zzyi9kh6xwf-python3.8-mitmproxy-5.2/lib/python3.8/site-packages/mitmproxy/tools/console/statusbar.py", line 169, in prompt_execute
    msg = p(txt)
  File "/nix/store/bavhjp7n9rkjzvzsavcw2zzyi9kh6xwf-python3.8-mitmproxy-5.2/lib/python3.8/site-packages/mitmproxy/tools/console/statusbar.py", line 115, in execute_command
    execute(txt)
  File "/nix/store/bavhjp7n9rkjzvzsavcw2zzyi9kh6xwf-python3.8-mitmproxy-5.2/lib/python3.8/site-packages/mitmproxy/tools/console/commandexecutor.py", line 18, in __call__
    ret = self.master.commands.execute(cmd)
  File "/nix/store/bavhjp7n9rkjzvzsavcw2zzyi9kh6xwf-python3.8-mitmproxy-5.2/lib/python3.8/site-packages/mitmproxy/command.py", line 266, in execute
    return self.call_strings(command_name, args)
  File "/nix/store/bavhjp7n9rkjzvzsavcw2zzyi9kh6xwf-python3.8-mitmproxy-5.2/lib/python3.8/site-packages/mitmproxy/command.py", line 252, in call_strings
    return self.commands[command_name].call(args)
  File "/nix/store/bavhjp7n9rkjzvzsavcw2zzyi9kh6xwf-python3.8-mitmproxy-5.2/lib/python3.8/site-packages/mitmproxy/command.py", line 122, in call
    ret = self.func(*bound_args.args, **bound_args.kwargs)
  File "/nix/store/bavhjp7n9rkjzvzsavcw2zzyi9kh6xwf-python3.8-mitmproxy-5.2/lib/python3.8/site-packages/mitmproxy/command.py", line 296, in wrapper
    return function(*args, **kwargs)
  File "/nix/store/bavhjp7n9rkjzvzsavcw2zzyi9kh6xwf-python3.8-mitmproxy-5.2/lib/python3.8/site-packages/mitmproxy/addons/export.py", line 149, in clip
    v = strutils.always_str(func(flow))
  File "/nix/store/bavhjp7n9rkjzvzsavcw2zzyi9kh6xwf-python3.8-mitmproxy-5.2/lib/python3.8/site-packages/mitmproxy/utils/strutils.py", line 26, in always_str
    return str_or_bytes.decode(*decode_args)
UnicodeDecodeError: 'utf-8' codec can't decode bytes in position 963-964: invalid continuation byte

mitmproxy has crashed!
Please lodge a bug report at:
	https://github.com/mitmproxy/mitmproxy

System Information

nix-shell (Nix) 2.3.7

Start with mitmproxy/addons/export.py at clip and follow the strutils.always_str call shown in the traceback. Reproduce the command against an 8.5 KB multipart/mixed response containing invalid UTF-8 bytes; done means exporting the raw response no longer crashes the console.