Multi-part form mime-types for each field

Hello,

When trying to submit a multi-part encoded form with several fields, there is currently no way to attribute specific mime-types to some or each of the fields of the form.

For example, in the same form, I might want to load a binary file in a field and impose a custom mime-type to it.

At the same time, I want to submit some text from a variable or directly entered and attribute it another custom mime-type (sometimes application/json, but not always).

Also, entering in a field some long text value with newlines is currently impossible.

Hopefully these features would not be too hard to add …