application/x-www-form-urlencoded or multipart/form-data?

In HTTP there are two ways to POST data: application/x-www-form-urlencoded and multipart/form-data. I understand that most browsers are only able to upload files if multipart/form-data is used. Is there any additional guidance when to use one of the encoding types in an API context (no browser involved)? This might e.g. be based on:

• data size
• existence of non-ASCII characters
• existence on (unencoded) binary data
• the need to transfer additional data (like filename)

I basically found no formal guidance on the web regarding the use of the different content-types so far.