This looks solid to me! Good work! Only one suggestion on the check box label.

What about discarding "Fill out the" in the label? It seems kinda repetitive when every item in the list has this text.

looking good! i think it would be nice to have a more complete screenshot (ie. without cutting off the top navigation bar) so we can get the full effect of the new banner, especially because it adds so much vertical space

can we use BEM for the class name?

is it possible to use one of the greys from reviewboard/static/rb/css/ui/colors.less ?

i think this class should follow the BEM naming conventions and maybe be clearer about what it's used for eg. rb-c-file-upload-container - file-area-container made me think that it's just used for showing a file or something like that

from the screenshot, the container looks about the same size as the thumbnails and might share some of the same properties right? are there constants defined for any of these properties that we can use?

i think we should be using BEM here for the class names too

i think the padding should use variables like in reviewboard/static/rb/css/ui/page-sidebar.less

missing documentation for this function

missing documentation

getting the element by appending the target value to the id selector and using jQuery to search for that feels kind of hacky - is it not possible to get the element from the event itself?

should value be "Click to browse"? (ie. first letter capitalized)

i think this would be better named as this._$editButton because it can be clicked and its role is button

i think this could just be called this._$editIcon since it's actually referring to the icon

from the other variables defined before this one, i think the id should be suffixed with -action to be consistent

i think this.$el.removeClass('placeholder-text') would be clearer

i think this.$el.addClass('placeholder-text') would be clearer

this.$el.removeClass('placeholder-text')

this.$el.addClass('placeholder-text')

I think the placeholder for new items should go at the end of the list. This is where new items show up, so it's like you're "filling in" the dashed box with the thing you're uploading.

There's some hilarious irony in these placeholders recommending limiting to 50 characters and wrapping to 72, then blasting past those limits.
Rather than suggesting those in the text (while we think they're good ideas, and enforce them for our stuff, not everyone does), let's just show by example what good lengths are.

We generally want to use parens for wrapping long things rather than continuation characters:

base_placeholder_text = (
    '... '
    '... '
    '...')

This use of a continuation character is correct, though.

Parens instead of \

Parens here too.

We use the name "Base" to indicate that the class will be inherited from. In this case I think "ReviewRequestFieldRequirements" might be more appropriate.
But I don't think we even need a new class for this--this isn't Java. A function (get_review_request_required_fields, maybe?) would be better.
Will need a docstring whichever way you go.

This should have a trailing comma at the end. Please also keep the keys in alphabetical order.

Please put this blank line back.

Should have two blank lines on either side of the top-level block.

Add a blank line here, please.

Since we're changing this, let's take the opportunity to move these to the new colors definitions.

@file-attachment-bg: #rb-ns-ui.colors[@white];
@file-attachment-overlay-bg: #rb-ns-ui.colors[@grey-90];
@file-attachment-overlay-border-color: #rb-ns-ui.colors[@grey-80];

...

@file-attachment-border-color: #rb-ns-ui.colors[@grey-60];

Don't need to add this line.

This should be in the imperative mood rather than passive ("Change the publish..." instead of "Changes")

Please wrap this line to 80 columns.

Please add a blank line between these two.
We also like to make sure any jQuery-type variable starts with $, so $checkbox for the name.

Please use single-quotes instead of double.

This can be a one-liner, and we can also use jQuery's eq method to simplify the selector:

this.$buttons.eq(0).prop(
    'disabled',
    this.requiredCheckboxes.length !==
        this.numberOfBoxesChecked);

There are a few things in here.
First, since we're in ES6, we can use template strings (note the backticks).
We should also not use the name "class", since it's a reserved word.
Finally, anything inside a call to gettext must be a literal. It can't be computed due to the way the gettext scanner works. Fortunately, there's a formatter we can use:

id: `${fieldName}-banner-checkbox`,
className: 'banner-checkbox',
label: interpolate(gettext('Fill out the %s field'),
                   [fieldName]),

This line can just be this._changePublishButtonState()

In ES6 files, we have a much better way of formatting text like this:

this._$attachments.prepend(dedent`
    <div class="rb-c-file-upload-container">
     <div>
      <p>Drop file attachments here</p>
      <p>or</p>
      <input type="button" id="select-file-for-review-action"
             value="Click to browse">
     </div>
    </div>
`);



That said, we should probably also wrap this in _.template and pass all the user-visible text in as variables that are wrapped in gettext() to enable localization of this UI.

In jQuery calls when creating a new (single) element, you don't need the closing tag.

Wrap to 80 columns please.

Wrap to 80 columns.

Add a blank line in here.

Wrap to 80 columns.

Add a blank line in here.

Wrap to 80 columns.

Perhaps I'm reading this wrong, but won't this actually write the placeholder text to the database? Don't we want to write an empty string instead?