Post data of cpr::File is different from a cpr::Buffer of the same file

[vancer0]

Description

When I try to upload a file within a Multipart with a Post request, the bytes are different when I use cpr::File and cpr::Buffer for the same file. Below is some example code I tried with a small PNG image.

// Using Buffer
std::vector<char> vec;
std::ifstream file("test.png");
std::copy(std::istream_iterator<char>(file), std::istream_iterator<char>(), std::back_inserter(vec));

cpr::Response res1 = cpr::Post(cpr::Url{"http://www.httpbin.org/post"},
                               cpr::Multipart{{"name", cpr::Buffer{vec.begin(), vec.end(), "test.png"}}});
std::cout << res1.text << std::endl;

// Using File
cpr::Response res2 = cpr::Post(cpr::Url{"http://www.httpbin.org/post"},
                               cpr::Multipart{{"name", cpr::File{"test.png"}}});
std::cout << res2.text << std::endl;

The output I get is this:

{
  "args": {}, 
  "data": "", 
  "files": {
    "name": "<some data>"
  }, 
  "form": {}, 
  "headers": {
    ...
    "Content-Length": "25654", 
    "Content-Type": "multipart/form-data; boundary=------------------------7XWq6nScPC3xKHD7Jjb9z2", 
    "User-Agent": "curl/8.6.0-DEV", 
    ...
  }, 
  "json": null, 
  "origin": "...", 
  "url": "http://www.httpbin.org/post"
}

{
  "args": {}, 
  "data": "", 
  "files": {
    "name": "<different data>"
  }, 
  "form": {}, 
  "headers": {
    ...
    "Content-Length": "28374", 
    "Content-Type": "multipart/form-data; boundary=------------------------btaObLMBqn5Laa0AYu2ytO", 
    "User-Agent": "curl/8.6.0-DEV", 
    ...
  }, 
  "json": null, 
  "origin": "...", 
  "url": "http://www.httpbin.org/post"
}

You can also see that the lengths are different. Also I should mention that if I save the vec buffer back again into a file, it is identical to the original.

Example/How to Reproduce

See code example

Possible Fix

No response

Where did you get it from?

vcpkg

Additional Context/Your Environment

• OS: Fedora Linux
• Kernel Version: 6.10.6
• cURL Version: 8.6.0 vcpkg