Add support for custom field types

Problem/Motivation

As part of an audio feature we created a custom field type "Audio" that is similar to the default "File" fields but only allows to upload audio files.
The current 7.x-2.0 module does not support this kind of custom field types because it uses a while loop to iterate through just two different file types (file and text). For other field types it leads to a series of indefined index errors.

Proposed resolution

The proposed solution is to add a default item to the while loop that provides a default value for the required parameters.