Mentioned the automatic set of "accept" based on "mimeTypes"

[javiereguiluz]

Fixes #12024.

[weaverryan]

I’m wondering if this should go on the FileType form field docs. I would be using that field and maybe wondering how to add the mime type stuff and this would tell me. But I think it’s fine either way.

[javiereguiluz]

Ryan, I thought about that ... but the File form field doesn't have an accept option (it's an HTML attribute defined via attr) so it's really hard to mention this feature there 😕

[javiereguiluz]

@OskarStark do you know why this is failing?

Warning, treated as error:

/home/travis/build/symfony/symfony-docs/reference/constraints/File.rst:251: 
WARNING: undefined label: forms-field-guessing (if the link has no caption the 
label must precede a section header)

The reference exists and I think the syntax is correct. Thanks.

[xabbuh]

@javiereguiluz Are you sure? I cannot find the reference.

[javiereguiluz]

It's strange. On my local computer I see this in the main forms.rst file:

But in the repository the reference is different: https://raw.githubusercontent.com/symfony/symfony-docs/4.4/forms.rst

.. _form-type-guessing:

[javiereguiluz]

OK, I've just rebased this PR to the latest 4.4 changes, so it should be OK now. Thanks.