If this is not possible with Sparkle (and I’m not sure if it is) you could try Jotform (online form maker) and develop the email form with that and import the form into a Embed widget etc. I haven’t tried it but it should work.
It is possible to build incredible form with Jotform
Or another possibility is to use CoffeCup Web Form Builder, which is a great app for building forms etc.
I just made a quick webpage using sparkle and created a upload form using web form builder for Mac. Works without issue when using the embed widget in Sparkle.
view the form on my test site here