POST request in Gatling

In case it helps anyone else, I was trying to set a custom boundary when Gatling already does this. Here's what solved my problem:

.exec(http("Post document: upload the file")
            .post("/api/data/files?revisionId=${documentRevisionId}&uploadType=read_only_file&fileType=application%2Fpdf&fileName=testdocument.pdf&fileSize=10080&copyToEditable=true") // ensure testdocument.pdf is in user-files/bodies
            .headers(Headers.headers_6)
            .formParamMap(Map(
                "resumableChunkNumber" -> "1",
                "resumableChunkSize" -> "1048576",
                "resumableCurrentChunkSize" -> "10080",
                "resumableTotalSize" -> "10080",
                "resumableType" -> "application/pdf",
                "resumableIdentifier" -> "${documentUuid}",
                "resumableFilename" -> "testdocument.pdf",
                "resumableRelativePath" -> "testdocument.pdf",
                "resumableTotalChunks" -> "1"))
            .bodyPart(RawFileBodyPart("file", "testdocument.pdf")
                .fileName("testdocument.pdf")
                .transferEncoding("binary")).asMultipartForm)

Although @Chuck's answer is correct, but the minimalistic code would be as simple as

  http("Create contentV2")
    .post("/some/path")
    .header("Content-Type", "multipart/form-data")
    .bodyPart(StringBodyPart("changeRequest", changeRequest)).asMultipartForm
    .bodyPart(StringBodyPart("payload", ElFileBody(filename))).asMultipartForm
    .check(status is 201)

Now we can have multiple things in place of StringBodyPart like RawFileBodyPart, ByteArrayBodyPart etc. more about it here