Files selected for <input type=file> should be validated against ‘accept’ attribute

[burg]

In chapter 14. Element Interaction, section 14.3 Element Send Keys:

Complete implementation specific steps equivalent to setting the selected files on the input. If multiple is true files are be appended to element’s selected files.

There is no provision to validate selected files against the 'accept' attribute. For example, this is defined here:

https://html.spec.whatwg.org/multipage/input.html#file-upload-state-(type=file)

It is also noted above that "User agents should prevent the user from selecting files that are not accepted by one (or more) of these tokens." I think it's worth referencing this or otherwise hooking into the validation that would be done by the UA in a real user scenario.

[andreastt]

For other elements we already rely on the validation done by the browser, but <input type=file> is special because we override the security permissions. Certainly in Firefox I don’t know what would happen if you upload a file type that it’s not supposed to accept.

[burg]

Safari/WebKit validates the selected files against supported values of accept=. I have some test cases that I can try to rewrite into WPT tests, but I don't have time for that at the moment.

[andreastt]

We also need prose in the specification that this is what the driver should be doing.

[whimboo]

I was already thinking about this problem when I worked on the patch for https://bugzilla.mozilla.org/show_bug.cgi?id=1448792 lately, but actually didn't file an issue for the spec. So it's good to see that this has been done now.

I can take care of adding tests once a decision has been made.