[Form][2.4] added an option for multiple files upload (closes #1400)

| Q             | A
| ------------- | ---
| Bug fix?      | [no]
| New feature?  | [yes]
| BC breaks?    | [no]
| Deprecations? | [no]
| Tests pass?   | [yes]
| Fixed tickets | [#1400]
| License       | MIT
| Doc PR        | [todo]