How to convert Apple's .textClipping files to plain text?
Futher googling about Apple's binary property lists lead me to try plutil  -convert xml1 note.textClipping which produces an XML file with the readable text of the snippet as UTF-8 text, then base64-encoded versions of the same text in UTF-16, mac-plain, and HTML.

Even after converting the binary to XML format, Finder still opens the file in its own viewer no matter what extension is set (e.g. .plist or .xml).

Running /bin/ls -al in the folder shows a @ next to the file name which indicates the file has extended attributes so it looks that they are causing the 'special' behavour in Finder.

This answer lead me to try 



xattr -l note.plist 


which reveals that some of the note's content is stored as an extended attribute key.

And finally, clearing all extended attributes turned the file into a 'normal' file, so now it opens with the associated app:

xattr -c note.plist 


The command above is for a single file, to strip extended attributes for all files in the current folder run:

xattr -rc .



  Extended attributes might be useful in some cases — for example, if you downloaded a file with safari or chrome the original URL will be stored as an extended attribute which you can see with xattr -l myfile.zip or right-clicking a file in Finder and selecting Get Info > The URL will be displayed under More Info > Where from. (Another reason not to use wget when you want to keep track of the file's origin.)