Allow File Attachments

[linchpinstudios]

Allow users to attach files to their forms and email them to the owner.

• Set a public $files = []; on the model to define files that should be attached.
• Setup UploadedFile object foreach $files on model
• Setup attachContent on ownerMail to get file content and attach it to the email.
• Added errors to the response of the form to return submission validation errors.

[Luuc15]

These changes use an attribute to be the field that defines the fields that have files that have to be emailed. It would be better to use a method for this so the attributes will always relate to a field.
Like a getUploadedFiles function. You can even let this function just loop all fields to check if it is an upload field.

Also the foreach loop reading all file fields uses attachContent function as if we can only upload one file per field. It would be better to be able process multi-fileupload as wel