`fields.remove` call with array of file inputs always removes last input in array, regardless of index argument

### Are you submitting a **bug report** or a **feature request**?
A bug report

### What is the current behavior?
Input arrays don't work with file inputs (``) as expected. When clicking any of the remove buttons in the supplied sandbox, the last entry in the list of inputs is removed, not the one selected. (the `fields.remove` function always removes the last input, regardless of the index argument). When substituting text input fields instead of file uploads, it works as expected. (see second sandbox link)

### What is the expected behavior?
The `fields.remove` function should remove the input at the index it was passed, not the last one.

### Sandbox Link

File inputs: https://codesandbox.io/s/silly-stonebraker-vyi6l
Text inputs: https://codesandbox.io/s/jovial-kalam-8esq7

### What's your environment?
```
"final-form": "4.16.1"
"final-form-arrays": "1.1.2"
"react": "16.8.6"
"react-dom": "16.8.6"
"react-final-form": "6.3.0"
"react-final-form-arrays": "3.1.0"
```

### Other information
None.