multipart/form-data is an encoding type that allows files to be sent through a POST. Quite simply, without this encoding the files cannot be sent through POST. If you want to allow a user to upload a file via a form, you must use this enctype.
We have made an API available that uses multipart/form-data for two reasons. Firstly it allows users of one of our older legacy systems to more easily migrate across to TruLinks. Secondly, it allows users to create websites that use this API directly from their UI