rest-assured/src/main/java/io/restassured/config/MultiPartConfig.java
public MultiPartConfig defaultCharset(Charset defaultCharset) {
        String charsetAsString = AssertParameter.notNull(defaultCharset, Charset.class).toString();
        return new MultiPartConfig(defaultControlName, defaultFileName, defaultSubtype, defaultBoundary, charsetAsString, true);
    }
public MultiPartConfig defaultCharset(Charset defaultCharset) {
        String charsetAsString = AssertParameter.notNull(defaultCharset, Charset.class).toString();
        return new MultiPartConfig(defaultControlName, defaultFileName, defaultSubtype, defaultBoundary, charsetAsString, true);
    }
Specify a default charset to use for multi-parts (default is US-ASCII). This affects the encoding of the multipart body (such as the
control name) but <i>not</i> the actual <i>content</i> (such as the a JSON or String document).
<p>
<b>NOTE:</b> This setting is <i>only</i> taken into account if {@link HttpClientConfig#httpMultipartMode(HttpMultipartMode)} is set to
something other than {@link HttpMultipartMode#STRICT} (which is the default). So if you want this setting to apply you also need to
explicitly change the multipart mode, for example:

<pre>
given().
config(RestAssuredConfig.config()
.httpClient(HttpClientConfig.httpClientConfig().httpMultipartMode(BROWSER_COMPATIBLE))
.multiPartConfig(multiPartConfig().defaultCharset("UTF-8"))).
when().
post("/x"). ..
</pre>
</p>
@param defaultCharset The default charset to use.
@return An updated MultiPartConfig