An OkHttp interceptor allowing you to override Content-Type for POST, PUT and PATCH requests

ContentTypeInterceptor.kt

/**
 * Overrides the HTTP Header Content-Type for POST, PATCH and PUT calls to the given [contentType]
 */
class ContentTypeInterceptor(private val contentType: String) : Interceptor {
    override fun intercept(chain: Interceptor.Chain): Response {
        val request = chain.request()
        val chainBuilder = request.newBuilder()
        request.body()?.let {
            val newBody = ForwardingRequestbody(it, contentType)
            when (request.method()) {
                "POST" -> chainBuilder.post(newBody)
                "PUT" -> chainBuilder.put(newBody)
                "PATCH" -> chainBuilder.patch(newBody)
                else -> { /* NO-OP */ }
            }
        }
        return chain.proceed(chainBuilder.build())
    }
}

class ForwardingRequestbody(private val sourceBody: RequestBody, contentType: String): RequestBody() {
    private val mediaType = MediaType.get(contentType)
    override fun contentType(): MediaType = mediaType
    override fun writeTo(sink: BufferedSink) = sourceBody.writeTo(sink)
}