API with matlab.net: Content-Range vs Content-Size

Dear all,

I am still a newbie in communication with servers and have the following problem:

I want to upload a file via Matlab API. For simple txt files, my matlab code works. Now, I want to upload a xlsx file (not a large file) and get the server response that the Content-Range is not matching the content size. I have no idea why this happens.

% Initizalization
import matlab.net.*
import matlab.net.http.*

%% Define the API endpoint with GUID
api_endpoint_upload = "https://XXX";
% Select file  
[FileName, dataFilePath] = uigetfile('*.*','Select file to upload');
filePath = fullfile(dataFilePath, FileName);
% Get file size
fileInfo = dir(filePath);
fileSize = fileInfo.bytes;
% Get binary data
fid = fopen(filePath, 'rb');
data = char(fread(fid)');     
fclose(fid);

% Define the headers for the API request
headers = matlab.net.http.HeaderField('x-api-key', 'YYYY');
headers(2) = matlab.net.http.HeaderField('Content-Type', 'application/octet-stream');
s1 = sprintf('form-data; filename=%s',FileName);
headers(3) = matlab.net.http.HeaderField('Content-Disposition', s1);
s2 = sprintf('bytes 0-%d/%d', fileSize-1, fileSize);
headers(4) = matlab.net.http.HeaderField('Content-Range', s2);

% Define and send post request:
request = matlab.net.http.RequestMessage('post',headers,data);
response = send(request2, URI(api_endpoint_upload));
response.Body.Data.message

% Get the response: Content-Range header size fileSize is not the same as
% the content size (fileSize + XY)

Answer 1

[Goutam]

Hi Simon,

The error is likely caused by how the file data is being read and encoded before transmission

In current code

data = char(fread(fid)');

This converts the raw file bytes into a MATLAB char array, which uses UTF-16 encoding , meaning each character occupies 2 bytes.

As a result:

- dir(filePath).bytes correctly reports the actual file size.

- But the HTTP payload becomes twice as large, since each byte is now represented by a 2-byte character.

- The server receives more data than expected, triggering a mismatch with the content-range header.

Here is a workaround that you may try :

1. Read the file as raw byte.

Use uint8 to preserve the original byte structure:

fid  = fopen(filePath, 'rb');
data = fread(fid, Inf, '*uint8').';  % row vector of raw bytes
fclose(fid);

2. Use MATLAB’s FileProvider for streaming.

Let MATLAB handle the file streaming and headers automatically:

import matlab.net.*
import matlab.net.http.*
import matlab.net.http.io.*
provider = FileProvider(filePath);  % streams the file
headers  = HeaderField('x-api-key', 'YYYY');
request  = RequestMessage('POST', headers, provider);response = send(request, URI(api_endpoint_upload));

Kindly refer to the documentation links below for reference :

• https://www.mathworks.com/help/releases/R2023b/matlab/matlab_prog/unicode-and-ascii-values.html
• https://www.mathworks.com/help/releases/R2023b/matlab/ref/fread.html
• https://www.mathworks.com/help/matlab/matlab_external/http-matlab-data-type-conversion.html
• https://www.mathworks.com/help/releases/R2023b/matlab/ref/matlab.net.http.io.fileprovider-class.html

Hope this helps

Best regards,

Goutam

[Simon]

Hi Goutam,

your suggestion worked out. Thank you!

Best regards,

Simon