Unable to manually reset value of `FileInputField`

[bedrich-schindler]

Reset using form's input is being implemented in #627. There is one more problem with reset and it is reset of its internal value. For native input, this can be achieved by setting value to null over its ref. This is not possible for FileInputField.

The aim of this issue is to introduce reset over its ref.

Aditional improvements to be discussed in the future.