Tool: PEP8 Style Checker
Processed Files:
    reviewboard/attachments/views.py
    reviewboard/urls.py



Tool: Pyflakes
Processed Files:
    reviewboard/attachments/views.py
    reviewboard/urls.py

 'HttpResponse' imported but unused

Col: 1
 E302 expected 2 blank lines, found 1

Col: 70
 W291 trailing whitespace

Col: 80
 E501 line too long (86 > 79 characters)

Should be one import line.

Two blank lines here.

"file_download" is what I'd prefer.
Can you also add a docstring explaining what this does? Make sure it's in the following form:
"""One-line summary.

Multi-line description.
"""

That should just go in the function arguments. No need to pull it out of kwargs.
In fact, you shouldn't have a kwargs here.
From the above changes, this would become:
def file_download(request, file_attachment_id):

The 404 would be publicly-visible, so if we were showing a message, we'd want to localize, make sure it's in sentence-casing, and be a bit more friendly with the error text.
Having custom text means that the default Review Board error page wouldn't be shown. So all around, we don't want to provide any of this. In fact, we don't want to return HttpResponseNotFound. Instead, raising Http404 will do the right thing.
There's a shortcut for all this. It will query, and if not found, raise Http404.
We can even simplify this further to ensure that we can only look up file attachments with file content. So, change all this to:
file_attachment = get_object_or_404(FileAttachment,
                                    pk=file_attachment_id,
                                    file__isnull=FalsE)

Use pk= instead of id= when referencing primary keys.

Should be "files/".
Can you also use "file_attachment_id", so it's explicit?

Tool: PEP8 Style Checker
Processed Files:
    reviewboard/attachments/views.py
    reviewboard/urls.py



Tool: Pyflakes
Processed Files:
    reviewboard/attachments/views.py
    reviewboard/urls.py

 'HttpResponseNotFound' imported but unused

 'ObjectDoesNotExist' imported but unused

Col: 50
 W291 trailing whitespace

Col: 56
 W291 trailing whitespace

Tool: Pyflakes
Processed Files:
    reviewboard/attachments/views.py
    reviewboard/urls.py



Tool: PEP8 Style Checker
Processed Files:
    reviewboard/attachments/views.py
    reviewboard/urls.py

I'd prefer not including the particular URL in here, because if we decide to move it, it's likely that this would get out of date.

Remove this blank line.

The second field in the tuple should be aligned with the first:

(r'^file...',
 'reviewboard.attachments...')

Missing a trailing period.

Should probably use file_attachment.get_absolute_url().

Tool: PEP8 Style Checker
Processed Files:
    reviewboard/attachments/views.py
    reviewboard/urls.py



Tool: Pyflakes
Processed Files:
    reviewboard/attachments/views.py
    reviewboard/urls.py

Tool: PEP8 Style Checker
Processed Files:
    reviewboard/attachments/views.py
    reviewboard/urls.py



Tool: Pyflakes
Processed Files:
    reviewboard/attachments/views.py
    reviewboard/urls.py