[Feature] File size validation

[mvi-x]

First time I post an Issue/Feature request. Please excuse me and let me know where I went wrong if this is a faux pas.

WTForms and flask-wtf provide excellent and easy to use validations. They are extremely useful to amateur programmers (like myself). However, for contact forms with attachments or file upload forms there is no easy file size validation and afaik each one writes their own solution. Custom solutions by noob programmers are ugly and possibly insecure. This seems like a front that could be improved and would make Flask and flask-wtf even more amateur-friendly and efficient for everyone.