This is not reIated to the last posts and I guess it’s too late already but since it’s somehow related to Gedcom 7: what about adding mimetypes to Notes in Gramps 6?
https://gramps-project.org/bugs/view.php?id=13176
(as Nick mentioned this could be added to Gramps 5.3 Importing basic html to StyledText - #6 by Nick-Hall 
)