Curl analog with URLRead/Execute?

To keep this example self-contained, we will use the HTTP introspection service at https://httpbin.org. This is a very handy site for verifying and debugging HTTP transactions.

First we will create a small JPG file:

Export["zot.jpg", Image[{{1}}]]

Then we will create a multi-part HTTP request that will post that file:

$req = HTTPRequest[
  "https://httpbin.org/anything"
  , <| "Method" -> "POST"
     , "Body" -> {"filedata" -> File["d:\\documents\\zot.jpg"]}
    |>
  ];

We can submit the request using URLRead and then inspect the body of the response. The introspection service returns a JSON description of the original request:

$resp = URLRead[$req];

$resp["Body"]

(*
{
  "args": {}, 
  "data": "", 
  "files": {
    "filedata": "data:image/jpeg;base64,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"
  }, 
  "form": {}, 
  "headers": {
    "Accept": "*/*", 
    "Accept-Encoding": "deflate, gzip", 
    "Content-Length": "653", 
    "Content-Type": "multipart/form-data; boundary=------------------------f14f77e69b501379", 
    "Host": "httpbin.org", 
    "User-Agent": "Wolfram HTTPClient 12.1", 
    "X-Amzn-Trace-Id": "Root=1-5eff61aa-a60799b6493fef5b401d85c4"
  }, 
  "json": null, 
  "origin": "52.0.14.116", 
  "url": "https://httpbin.org/anything"
}
*)

Alternatively, we could use URLExecute which automatically imports the JSON response body as nested lists of rules:

URLExecute[$req]

(*
{ "args" -> {}
, "data" -> ""
, "files" -> {"filedata" -> "data:image/jpeg;base64,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"}
, "form" -> {}
, "headers" ->
  { "Accept" -> "*/*"
  , "Accept-Encoding" -> "deflate, gzip"
  , "Content-Length" -> "653"
  , "Content-Type" -> "multipart/form-data; boundary=------------------------61723d168bdcb569"
  , "Host" -> "httpbin.org"
  , "User-Agent" -> "Wolfram HTTPClient 12.1"
  , "X-Amzn-Trace-Id" -> "Root=1-5eff6281-e0a4c5e08b883fc0f0437148"
  }
, "json" -> Null
, "origin" -> "52.0.14.116"
, "url" -> "https://httpbin.org/anything"
}
*)

The result returned by the introspection service makes it easy for us to verify that these results are equivalent to using curl:

curl https://httpbin.org/anything -F filedata=@d:\documents\zot.jpg

{
  "args": {},
  "data": "",
  "files": {
    "filedata": "data:image/jpeg;base64,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"
  },
  "form": {},
  "headers": {
    "Accept": "*/*",
    "Content-Length": "653",
    "Content-Type": "multipart/form-data; boundary=------------------------16397e5167d6e462",
    "Host": "httpbin.org",
    "User-Agent": "curl/7.55.1",
    "X-Amzn-Trace-Id": "Root=1-5eff6094-0034631c571243182edd4b3c"
  },
  "json": null,
  "method": "POST",
  "origin": "52.0.14.116",
  "url": "https://httpbin.org/anything"
}