Multipart form file upload, Nginx is "eating" it and not passing it on to the handler - file-upload

Question

I am new to nginx, but am familar with low-level HTTP and apache. When I try to do a multipart/form file upload, nginx writes some of the client request body to disk, but never finishes and it never passes it to the down/upstream script.
My specific setup is I have nginx with /dyn redirected to localhost:1337, where a node.js instance is listening. It works... except for the file upload handler. Also in the config is a /debug which is redirected to localhost:1338, which goes to a simple dump server.
I changed the error log handling to 'info'. It reports storing the client body to a file and when I exampline it is almost as I expected:
--boundary_.oOo._MjM5NzEwOTkxMzU2MjA0NjM5MTQxNDA3MjYwOA==
Content-Type: image/jpeg
Content-Disposition: form-data; name="file"; filename="dccde7b5-25aa-4bb2-96a6-81e9358f2252.jpg"
<binary data, ~89k>
The problem with this file is too short, only 81,920 bytes when the file is 88,963 bytes, it should be 88,963 + the header above.... But that is literally only half of it. There are 2 files (about the same size) that are coming in, so i would expect that file to be about ~160k. What nginx is doing is reassigning the request's http-level Content-length then passes that header on to the script, that's it and of course my script complains that it never finds --boundary_.oOo._MjM5NzEwOTkxMzU2MjA0NjM5MTQxNDA3MjYwOA== When I do the same request to my debug service without nginx in the middle, it corrrectly sends the data, anf the http Content-length is an apropriate 186943 bytes (both files are around 80k, so this makes sense)
My nginx config is default aside from what I've mentioned here.
Edit: after some more experimenting, all files in the client body directory are 81920 bytes