My thanks to all three of you that have responded to my query so far.

Dmapr: Yes, once I knew what the output was being directed to I found that solution. So that problem solved.

The file name I am using, mimetype, and its content, application/epub+zip, are correct so it must have something to do with how I am doing the compression. I have always used 7-Zip File Manager according to the following steps:

add mimetype to create the archive using Compression Level – Store

add OEBPS folder (containing all book content files) to archive using Compression Level – Normal

add META-INF folder (holds container.xml) to archive using Compression Level – Normal

Change file name to epub extension (from zip)

This has always produced an epub file readable as far as I am aware by all devices; personally verified Sony 900, Sony 505, Calibre, Firefox epub plugin, and Adobe reader.
This is why I have never worried over that message before, but if there is another suggestion for how to do the compression to produce a completely 'legit' file I have open eyes.