[api] Send Content-Disposition: inline for text files

Without this fix, text files such as _log are served with
`Content-Disposition: attachment`, which makes Firefox prompt the user to
download the file rather than simply showing the text within the browser.

src/api/app/controllers/application_controller.rb

@@ -374,6 +374,7 @@ def volley_backend_path(path)
       end
     end
     opts[:length] = @volleyfile.length
+    opts[:disposition] = 'inline' if opts[:type] == 'text/plain'
     # streaming makes it very hard for test cases to verify output
     opts[:stream] = false if Rails.env.test?
     send_file(@volleyfile.path, opts)