Add command for file uploads (separate out from Element Send Keys)

[whimboo]

This is a request which came up during discussions on issue #1230. Especially due to broken behavior of drivers when it comes to hidden file upload input elements.

As agreed during TPAC (minutes) the proposal from @andreastt should be implemented. For details see #1230 (comment)

[andreastt]

@burg had a request over in #1268 about adding a duplicate file check to Element Send Keys. @whimboo asked if we wanted to still add that to Element Send Keys, and I suspect the answer is that we don’t.

But adding a check for duplicate files to the new file upload command seems like a unanimously good idea, whilst also not breaking any backwards compatibility with Element Send Keys.

We can have this two ways:

1. Return an error when the remote end discovers two of the files have identical paths.

2. Deduplicate duplicate path entries.

[etanol]

Quick question. Is this about adding a new /session/{id}/file endpoint to be able to attach files in remote WebDriver sessions? Like what ChromeDriver supports for this non-W3C endpoint.

Imagine the WebDriver client is running in host A and the server in host B. Making the browser in host B upload a file (as in <input type="file">) that is in host A is quite cumbersome without the file endpoint.

Should I open a different issue for such a request?

Thanks.