Enhancement: Check the file size on upload for the file field

[JarrydLong]

Related to this ticket: https://www.paidmembershipspro.com/forums/topic/file-upload/

What currently happens:

• We check the file type and if allowed, upload, else throw an error

What should happen:

• Add in a field when registering the field (if left empty, get the servers max_upload_size value
• Check if the file meets that requirement upon upload, else throw an error