Sending multipart/mixed content with Postman Chrome extension

I was facing this problem too. Short answer: remove the Content-Type header from your Postman request.

The long story is that the Content-Type for a multipart request should be rather special -- it should look kind of like this:

multipart/form-data; boundary=----WebKitFormBoundaryzeZR8KqAYJyI2jPL

The problem is that the boundary is important and it needs to exactly match the boundary used to separate the files being uploaded. The solution is simple: do not specify a Content-Type! When you upload files, Postman will automatically append the above content type for you, except the boundary will be filled in with whatever Postman or Chrome is using to separate the multipart content.

You can verify this behavior by using Chrome developer tools (within Postman) to examine the Content-Type header being added, in addition to the Content-Disposition headers of the multipart data, which are also a pain to construct manually (and impossible within Postman).

Note: My answer is a solution for those who need a multipart/form-data answer. The OP was looking for a multipart/mixed solution. My answer will not suffice in this scenario. That being said, it seems a lot of people just want the multipart/form-data solution, so I will leave my answer here.

Left this comment on: https://github.com/postmanlabs/postman-app-support/issues/1104

Ninja update: Not sure if this will help anyone else but there is a workaround for a specific scenario where you have multiple file types / content types being uploaded in a single multipart POST request.

1. Set the Header Content-Type to multipart/mixed.
2. Select the form-data option in Body.
3. Convert all of your items into files. String content should become a text file, etc.
4. Add each file by selecting file, adding a key name.

This approach doesn't require actually manually specifying each Content-Type or Content-Disposition. The trick here was to serialize all relevant content into a persistent file type. You can ignore the "convert it into a file" step if it's text :) Hope that helps someone!