Automatically convert data with files to FormData

[reinink]

In order to upload files with Inertia, you have to convert your data object to a FormData object:

var data = new FormData()
data.append('first_name', first_name || '')
data.append('last_name', last_name || '')
data.append('email', email || '')
data.append('password', password || '')
data.append('photo', photo || '')

Inertia.post('/users', data)

However, this is annoying to have to do. I think it would be great if Inertia could detect if the data object includes files, and if it does, convert it to a FormData object.

// automatically  if data includes files
Inertia.post('/users', data)

While implementing this, it would be good to tackle this issue as well: #236

Reference

Laravel Jetstream already does this automatically (see here, here and here).

Axios also does some work to detect if a file is included (see here).

Booleans

When submitting data as a FormData object, the data is no longer json, meaning any boolean values will become strings values ("true" and "false"). I think we can simply convert these to 1 or 0.

Multi-dimensional objects

In addition to converting booleans, we'll also have to convert multi-dimensional objects. Something like this:

{
    user: {
        first_name: 'Jonathan',
        last_name: 'Reinink',
    }
}

Becomes:

user[first_name]=Jonathan
user[last_name]=Reinink