Improve multipart features for httpPost and httpUpload

Feature Request

With the current API, some scenarios of multipart interaction are not friendly and require what imho are hacks.

Description

I am working with CMS API and there’s a API call with requires uploading a document alongside metadata in JSON. This is easily done in curl with:

curl "https://api.hostname/v1/documents" \
-X POST \
-H 'Content-Type: multipart/form-data' \
-H "Authorization: Bearer $token" \
-H "Repository-Id: $repo_id" \
-F metadata="{
    \"description\": \"Test document\",
    \"categories\": [{...}]
	};type=application/json" \
-F file=@file_path

I only got this to work with the following code

    val (request, response, result) = url.httpUpload(
//            parameters = listOf("metadata" to """{"description":"test document from Kotlin"};type=application/json""")
    )
            .dataParts { request, url ->
                listOf(
                        DataPart(File("src/main/resources/rust-logo-blk.svg"), "file"),
                        DataPart(File("src/main/resources/doc.json"), "metadata","application/json")
                )
            }
            .header("Authorization" to "Bearer $token")
            .header("Repository-Id" to repoId)
            .responseString()

As you can see this requires storing the JSON in a file. That’s because I could not find any way to set the Content-Type for the part other than using DataPart. Using parameters as seen in the commented line sets it as text/plain causing the service to return an error.

I also tried something like

    val (request, response, result) = url.httpPost(listOf(
            "metadata" to """{"description":"test document from Kotlin"}""",
            "file" to DataPart(File("resources/rust-logo-blk.svg"))))
            .header("Content-Type" to "multipart/form-data")
            .header("Authorization" to "Bearer $token")
            .header("Repo-Id" to repoId)
            .responseString()

Proposed Solution

Imho, the naming upload is confusing for this cases but is very useful for simple 1 file uploads. I’d advocate to:

1. Create a httpMultipart similar to httpPost that offers the ability to pass any kind of object, not only files and set the Content-Type. Or allow htttPost to set Content-Type to multipart so that it manages the format internally (a la curl).
2. Extend DataPart to support any data (String, Inputstream, …)

Probably, with only point 2 would be enough to support a cleaner method with httpUpload.

Alternatives I’ve considered

Current solution does not allow to generate metadata Json on-the fly, which is not valid for a dynamic system. Also, the dataParts closure element seems cumbersome to me and does not help readability. I prefer simpler/cleaner httpPost syntax. But this is a more personal thing.

Additional context

I noticed that when no Content-Type is set in the DataPart, the header is added empty, maybe this could be just removed.

[abelsromero]

Can’t promise, but I’ll have a look at it asap this week.

[abelsromero]

Cool! Thanks a lot, do not hesitate to mention me if you need a beta tester.