Set the filename in Content-Disposition for inline files

[jsma]

In 2.11, the sendfile util was changed to serve PDFs inline. In the process, this prevented the filename from being set so when users go to save the PDF, the filename would be blank in the save dialog. This PR always sends the filename in the "Content-Disposition" header whether the file is served as inline or attachment so the save dialog will have a sensible default value.

[squash-labs]

Manage this branch in Squash

Test this branch here: https://greenlightgoset-filename-for-i-rxmo0.squash.io

[lb-]

@jsma this looks good in principle, but could you please update the unit tests for this + add some for this behaviour.

Additionally, there has been some reformatting work done since this PR was created so you will need to run black to reformat the changes.

[gasman]

@lb- Unless I'm missing something, I think the existing test here does the job - the change is to add a filename to the header that wasn't there before, and the test has been updated to reflect that.

[lb-]

@gasman good call, took another look and yeah I see that now.

I haven't validated locally yet and still needs a rebase. Might be able to take a look at that tomorrow but feel free to pick it up if you want.

[lb-]

Looks good @jsma I have tested locally and it does what it says on the box - thanks so much!

[lb-]

Merged in via 4a7fb00