Ensure tiddlers with fieldnames containing colons don't get saved in .tid file format

Prompted by discussion over at https://github.com/Jermolene/TiddlyWiki5/pull/5708#issuecomment-862399985

core/modules/utils/filesystem.js

@@ -228,6 +228,7 @@ exports.generateTiddlerFileInfo = function(tiddler,options) {
 			hasUnsafeFields = hasUnsafeFields || /[\x00-\x1F]/mg.test(value);
 			hasUnsafeFields = hasUnsafeFields || ($tw.utils.trim(value) !== value);
 		}
+		hasUnsafeFields = hasUnsafeFields || /:/mg.test(fieldName);
 	});
 	// Check for field values 
 	if(hasUnsafeFields) {