Custom background uploader doesn't highlight max upload size

[Denis-de-Bernardy]

When uploading a file from the custom background handler, you cannot tell in advance if it'll succeed or not if the file is too large. WP should advertise the max size, and the available space where relevant (see also #13765).

[ocean90]

To handle error messages better we need a bigger change for custom background (and custom header, see #13720). We should do this in 3.1.

[nacin]

Hmm. I don't know if I really care about this. Or, if necessary, it might be good if we just let the media manager do its thing and skip the idea of a direct type="file" input.

[ocean90]

Duplicate of #22148.

[SergeyBiryukov]

This ticket is not about Customizer, so I guess #22148 won't handle this (not until #25569 happens).

[ocean90]

Oh, you're right. Now the comment by nacin makes sense. I think we should ditch the type="file" input then.

[celloexpressions]

Fixed by [30309], and [30459].