SOAP APIs that require attachments may use MTOM (Message Transmission Optimization Mechanism) for more efficient transmission of binary content in a request. Review the specific API documentation to see whether MTOM can be used, or whether the attachment content must be included as base64 encoded content within the SOAP message.
MTOM messages are sent as a multipart MIME package, with the SOAP package as one part and the binary content in other parts. This allows a smaller request size, as otherwise the attachment must be sent as a string encoded using base64 technique, which increases the attachment size by a factor of 1.33x.
Further documentation on how to use MTOM can be found at: