`attach_file` not working with remote browser

[langalex]

Running a remote browser on a different host (e.g. Docker) and calling attach_file results in a file not found error. It would be useful to either somehow support file uploads or show an error message that they not supported on remote browsers.

[twalpole]

I'm sorry I haven't had any time to give apparition the attention it needs, and have prioritized the small amount of free time I have had to Capybara. I'm willing to look at any PRs here but beyond that don't know when I'll be able to put more time towards apparition.

If you want to submit a PR for remote file uploads I'll try and find time to review