Docs(formula): document file property type

[texastoland]

My use case was working with daily notes as dates (instead of their file creation date). I knew about parseDate(<filename>) but not how to query filename from prop('File'). Discussions about the metadata attribute led me to https://github.com/Make-md/makemd/blob/main/src/shared/types/metadata.ts. tasks was also recently added to Markdown in https://github.com/Make-md/makemd/blob/main/src/shared/types/metadata.ts. Until formulas support autocomplete it'd help to document in https://www.make.md/docs/References/Formula 🤓

[FlySandwich4]

Vote for the feature. I was trying to add a mtime to the column but it is not working. Seems only the default 'ctime' column working