modules/spring-mock-mvc/src/main/java/io/restassured/module/mockmvc/specification/MockMvcRequestSpecBuilder.java
public MockMvcRequestSpecBuilder addMultiPart(String controlName, File file) {
        spec.multiPart(controlName, file);
        return this;
    }
public MockMvcRequestSpecBuilder addMultiPart(String controlName, File file) {
        spec.multiPart(controlName, file);
        return this;
    }
Specify a file to upload to the server using multi-part form data uploading with a specific
control name. It will use the content-type <tt>application/octet-stream</tt>.
If this is not what you want please use an overloaded method.

@param file        The file to upload
@param controlName Defines the control name of the body part. In HTML this is the attribute name of the input tag.
@return The request specification