接口说明：

webapi附件上传接口说明 (kingdee.com)

在物料上加了2个附件字段，一个单附件，一个多附件，ID分别是F_UNW_Attachment_83g、F_UNW_Attachments_qtr

K3CloudApiClient apiClient;
private string url = "http://localhost/k3cloud/";
private string dbid = "65b8.......d06";
private string userName = "demo";
private string password = "888888";
private int lcid = 2052;

void Main()
{
	apiClient = new K3CloudApiClient(url);
	var isLoginOk = apiClient.Login(dbid, userName, password, lcid);
	if (!isLoginOk) return;

	var javaScriptSerializer = new System.Web.Script.Serialization.JavaScriptSerializer();

	//上传附件
	var uploadResultJson1 = apiClient.UploadFile(javaScriptSerializer.Serialize(new
	{
		FileName = "text1.txt",
		IsLast = true,
		SendByte = Convert.ToBase64String(Encoding.UTF8.GetBytes("content1 " + DateTime.Now))
	}));
	var fileId1 = (javaScriptSerializer.Deserialize<Dictionary<string, object>>(uploadResultJson1)["Result"] as Dictionary<string, object>)["FileId"].ToString();

	var uploadResultJson2 = apiClient.UploadFile(javaScriptSerializer.Serialize(new
	{
		FileName = "text2.txt",
		IsLast = true,
		SendByte = Convert.ToBase64String(Encoding.UTF8.GetBytes("content2 " + DateTime.Now))
	}));
	var fileId2 = (javaScriptSerializer.Deserialize<Dictionary<string, object>>(uploadResultJson2)["Result"] as Dictionary<string, object>)["FileId"].ToString();


	//获取附件信息
	var fileInfos = apiClient.ExecuteBillQuery(javaScriptSerializer.Serialize(new
	{
		FormId = "BOS_FileServerFileInfo",
		FieldKeys = "FFileId,FFileName,FFileSize",
		FilterString = $"ffileid in ('{fileId1}','{fileId2}')"
	}))
	.Select(r => new { fileId = r[0], fileName = r[1], fileSize = r[2] }).ToList();


	//保存附件
	var FNumber = "A" + DateTime.Now.ToString("yyyyMMddHHmmssfff");
	var FName = FNumber;
	var saveResult = apiClient.Save("BD_MATERIAL", javaScriptSerializer.Serialize(
	new
	{
		NeedReturnFields = new string[] { "F_UNW_Attachment_83g", "F_UNW_Attachments_qtr" },
		Model = new
		{
			FNumber,
			FName,
			//单附件字段
			F_UNW_Attachment_83g = fileInfos[0].fileId,
			//多附件字段
			F_UNW_Attachments_qtr = fileInfos.Select(fi => new
			{
				ServerFileName = fi.fileId,
				FileName = fi.fileName,
				FileBytesLength = fi.fileSize
			})

		}
	}));


	//修改附件
	var billId = (javaScriptSerializer.Deserialize<Dictionary<string, object>>(saveResult)["Result"] as Dictionary<string, object>)["Id"].ToString();
	var modifyResult = apiClient.Save("BD_MATERIAL", javaScriptSerializer.Serialize(
	new
	{
		NeedUpDateFields = new string[] { "F_UNW_Attachment_83g", "F_UNW_Attachments_qtr" },
		NeedReturnFields = new string[] { "FID", "FNumber", "F_UNW_Attachment_83g", "F_UNW_Attachments_qtr" },
		Model = new
		{
			FMATERIALID = billId,
			//单附件字段
			F_UNW_Attachment_83g = fileInfos[1].fileId,
			//多附件字段
			F_UNW_Attachments_qtr = new object[]{
			new
			{
				ServerFileName = fileInfos[1].fileId,
				FileName = fileInfos[1].fileName,
				FileBytesLength = fileInfos[1].fileSize
			}}

		}
	}));

}

WebApi报文：

上传附件1 http://[IP]/K3Cloud/Kingdee.BOS.WebApi.ServicesStub.DynamicFormService.UploadFile.common.kdsvc
{"FileName":"text1.txt","IsLast":true,"SendByte":"Y29udGVudDEgMjAyNC81LzIyIDE0OjE5OjAy"}

上传结果：
{"Result":{"ResponseStatus":{"IsSuccess":true,"Errors":[],"SuccessEntitys":[],"SuccessMessages":[],"MsgCode":0},"FileId":"0e6df974470d4bc29e697e5b6c6c20ab","Message":""}}


上传附件2 http://[IP]/K3Cloud/Kingdee.BOS.WebApi.ServicesStub.DynamicFormService.UploadFile.common.kdsvc
{"FileName":"text2.txt","IsLast":true,"SendByte":"Y29udGVudDIgMjAyNC81LzIyIDE0OjE5OjAy"}

上传结果：
{"Result":{"ResponseStatus":{"IsSuccess":true,"Errors":[],"SuccessEntitys":[],"SuccessMessages":[],"MsgCode":0},"FileId":"647a195dd6dd4f20bbc655985c51a50c","Message":""}}



获取附件信息 http://[IP]/K3Cloud/Kingdee.BOS.WebApi.ServicesStub.DynamicFormService.ExecuteBillQuery.common.kdsvc
{"FormId":"BOS_FileServerFileInfo","FieldKeys":"FFileId,FFileName,FFileSize","FilterString":"ffileid in (\u00270e6df974470d4bc29e697e5b6c6c20ab\u0027,\u0027647a195dd6dd4f20bbc655985c51a50c\u0027)"}

获取附件信息结果
[["0e6df974470d4bc29e697e5b6c6c20ab","text1.txt",27],["647a195dd6dd4f20bbc655985c51a50c","text2.txt",27]]


保存单据：
{"NeedReturnFields":["F_UNW_Attachment_83g","F_UNW_Attachments_qtr"],"Model":{"FNumber":"AX20240522141902286","FName":"AX20240522141902286","F_UNW_Attachment_83g":"0e6df974470d4bc29e697e5b6c6c20ab","F_UNW_Attachments_qtr":[{"ServerFileName":"0e6df974470d4bc29e697e5b6c6c20ab","FileName":"text1.txt","FileBytesLength":27,"FileLength":27},{"ServerFileName":"647a195dd6dd4f20bbc655985c51a50c","FileName":"text2.txt","FileBytesLength":27,"FileLength":27}],"FAttachment":"0e6df974470d4bc29e697e5b6c6c20ab"}}

保存结果：
{"Result":{"ResponseStatus":{"IsSuccess":true,"Errors":[],"SuccessEntitys":[{"Id":152058,"Number":"AX20240522141902286","DIndex":0}],"SuccessMessages":[],"MsgCode":0},"Id":152058,"Number":"AX20240522141902286","NeedReturnData":[{"FMATERIALID":152058,"F_UNW_Attachment_83g":"0e6df974470d4bc29e697e5b6c6c20ab","F_UNW_Attachments_qtr":"[{\"ServerFileName\":\"0e6df974470d4bc29e697e5b6c6c20ab\",\"FileName\":\"text1.txt\",\"FileBytesLength\":27,\"FileLength\":27},{\"ServerFileName\":\"647a195dd6dd4f20bbc655985c51a50c\",\"FileName\":\"text2.txt\",\"FileBytesLength\":27,\"FileLength\":27}]"}]}}