Uploading files via Contact form

[githubjeka]

Added missed multipart/form-data
Added missed multiple name attribute
Closing DIV tag matches nothing

Q	A
Is bugfix?	✔️
New feature?	❌
Breaks BC?	❌

[arogachev]

Tested it with running app, the error

`TypeError`
Cannot assign string to property `App\Contact\ContactForm::$attachFiles` of type `?array `

is gone.

But now the handling needs to be fixed too:

`Error`
Call to a member function `getStream()` on array 

Can be done in a separate PR though. Maybe #438 is related.

Thanks.