I’ve just noticed that if a field in a tiddler begins or ends with a space, the fielder will be saved as json in my tw server setup. The field I remarked it and tested it after spotting this is neither “title”, “tags” or “filetype”. Something like personal “use” or “toto” triggers it.
Here is my $:/config/FileSystemExtensions
tiddler:
[field:type[application/x-tiddler-dictionary]then[.dic]]
[field:type[text/vnd.tiddlywiki]then[.tw5]]
[field:type[]then[.tw5]]
[field:type[text/plain]then[.txt]]