fix(templates): Throw 400 when posting unexpected file upload name

[jan-law]

Fixes #601

When posting a file upload with a form attribute that doesn't match the name template, the TemplatesPostHandler will return an HTTP 400.

[jan-law]

The unit tests shouldSendNotifcationOnTemplateDeletion and shouldProcessGoodRequest are failing because the requests include "unused" file uploads. My patch is stricter than the original implementation and doesn't allow extra form attributes in the request. Can I safely remove the following lines from those tests?

        FileUpload upload2 = Mockito.mock(FileUpload.class);
        Mockito.when(upload2.name()).thenReturn("unused");
        Mockito.when(upload2.uploadedFileName()).thenReturn("/file-uploads/wxyz-9999");

[andrewazores]

That seems fine.