In Mailman 2.1.1, the mass subscription page has this text under the textbox, introducing the "Choose File" button

....or specify a file to upload:

I just talked with a not-dumb list owner who selected an Excel file, producing a wonderful list of 22 non-deletable, non-modifiable entries, which had addresses jumbled together with strings like %0F%00%00.

After that conversation, I would suggest that the text read

....or specify a plain text file to upload:

Fortunately, the list in question was new, so we elected to blow it away and try again.

--John