Here's what I have done:
activated the nearly-RC3 version of mailpress 5.0 (after duly copying,
backing up, deactivating, deleting, etc.). I re-activated a bunch of
add-ons (really nice interface for that, by the way - I was a bit
confused because of the change from prior versions, but once I
realized what was going on, I really like it!)
I added the category-27.xml file to the newsletters/categories folder.
I created a new singlecat-27.php file in my theme folder. It's an
exact copy of my single.php file.
I have tested using a draft post with a picture and text in it, which
is in the "new" category (id=27). In the Mailpress test box I have
selected the newsletter "per post 'category 27'" template. I get an
email with only a header (the same header that I get in all of my
emails using Mailpress, including the link to "view on our website"),
but there is no text or picture of any content of the post. The
subject of the email is "New post in category 27 : creating flim
flam."
So, clearly, it's not using my template file.
When testing using different newsletter category templates, the
content of the post come through just fine.
I should also point out: I notice that now my "new" category is, in
all things Mailpress, called "category 27." In the newsletter
subscription admin page, it's listed as "category 27" and in the test
email option, there is no option for using the "new" category
template, but instead the templates for "category 27." I am guessing
that is because that is how they are defined in the category-27.xml
file.