Maybe this can help you:

https://community.appeon.com/index.php/qna/q-a/httpclient-post-multipart-form-data-request?limitstart=0#reply-5419

https://community.appeon.com/index.php/qna/q-a/how-to-send-files-by-httpclient-object?limitstart=0#reply-24137

https://community.appeon.com/index.php/qna/q-a/how-to-send-https-post-with-xml-the-body

https://community.appeon.com/index.php/qna/q-a/using-rest-client-to-send-file

this is C# example which provided by API team

Here is an example. The function ReadBinaryFile is a function in the same object that uses FileOpen/FileReadEx/FileClose to read the file into a blob.

HttpClient lnv_Http
Constant Long BlobPartLen = 65536
Blob lblb_PostData, lblb_PostPart
Integer li_rc, li_PartNum
Long ll_StatusCode, ll_Bytes
String ls_ServerURL, ls_RecvData
String ls_PathName, ls_FileName, ls_StatusText

li_rc = GetFileOpenName("Select a File", ls_PathName, ls_FileName)
If li_rc < 1 Then Return

mle_response.Text = ""
cb_openfile.Enabled = False
FileDelete(is_FileName)

Yield()

SetPointer(HourGlass!)

// read the file
ll_Bytes = ReadBinaryFile(ls_PathName, lblb_PostData)
If ll_Bytes = 0 Then
	MessageBox(this.Text, &
					"ReadBinaryFile Failed!", StopSign!)
	Return
End If

// set the URL
ls_ServerURL = "http://localhost/pbhttp/webservice.pbh"

// create the object
lnv_Http = Create HttpClient

// set headers
lnv_Http.SetRequestHeader("FileName", ls_FileName)
lnv_Http.SetRequestHeader("Content-Length", String(Len(lblb_PostData)))

// send the request
li_rc = lnv_Http.PostDataStart(ls_ServerURL)
If li_rc = 1 Then
	// post the data in parts
	do while li_rc = 1
		lblb_PostPart = BlobMid(lblb_PostData, li_PartNum * BlobPartLen + 1, BlobPartLen)
		li_rc = lnv_Http.PostData(lblb_PostPart, BlobPartLen)
		li_PartNum ++
	loop
	lnv_Http.PostDataEnd()

	// get results
	ll_StatusCode = lnv_Http.GetResponseStatusCode()

	If ll_StatusCode = 200 Then
		// HTTP status 200 = OK
		lnv_Http.GetResponseBody(ls_RecvData)
		mle_response.text = ls_RecvData
	Else
		lnv_Http.GetResponseBody(ls_RecvData)
		mle_response.text = ls_RecvData
		ls_StatusText = lnv_Http.GetResponseStatusText()
		MessageBox("SendRequest Error #" + String(ll_StatusCode), &
						ls_StatusText, StopSign!)
	End If
Else
	MessageBox(this.Text, &
					"PostDataStart Error: " + String(li_rc), StopSign!)
End If

// destroy the object
Destroy lnv_Http

Hi Shameem,

It looks like it is just a simple multi-part form upload which you'll find heaps of information about.  The link below has almost the exact code you need to post a file as a multipart form. 

https://community.appeon.com/index.php/qna/q-a/snap-develop-api-file-upload-download#reply-28996

Normally for authentication a form upload would use Basic Auth, but for this one it looks like they just add a couple of extra headers for username and password.  You'll need to add that to the code in the example where the other header is set add these two lines (with your username and password).

lnv_httpClient.SetRequestHeader("username", "user")

lnv_httpClient.SetRequestHeader("password", "pass")

You'll also need to rename the uploadedFile in this statement to file and maybe also change invoice.pdf to the actual filename of the invoice

blb_file = blob( 'Content-Disposition: form-data; name="uploadedFile"; filename="invoice.pdf"~r~n' + &
                        "Content-Type: application/pdf~r~n~r~n", EncodingUTF8!) + &
                        blb_file

You'll also need to set ls_endpoint to the url of your api

ls_endpoint = 'http://localhost:5000/api/file/upload'