Enabling binary media types breaks Option POST call (CORS) in AWS Lambda - asp.net-core

Question

New to AWS..
We have a .NET Core Microservice running on a serverless aws instance as lambda functions.
Our Controller looks like this
[Route("api/[controller]")]
[ApiController]
public class SomeController : ControllerBase
{
[HttpGet()]
[Route("getsomedoc")]
public async Task<IActionResult> GetSomeDoc()
{
byte[] content;
//UI needs this to process the document
var contentDisposition = new System.Net.Http.Headers.ContentDispositionHeaderValue("attachment");
contentDisposition.FileName = "File Name";
Response.Headers[HeaderNames.ContentDisposition] = contentDisposition.ToString();
return File(content, "application/octet-stream");
}
[HttpPost()]
[Route("somepost")]
public async Task<IActionResult> SomePost()
{
return null;
}
}
URL's
{{URL}}/getsomedoc
{{URL}}/somepost
We have enabled 'Binary Media Types' in AWS package settings to / for the getsomedoc to work otherwise it was returning the byte array back instead of the file.
But this is breaking our 'somepost' call when UI is accessing the API using
Method: OPTIONS & Access-Control-Request-Method as POST
When we remove the binary media type the 'somepost' starts working.
Looking for suggestions as why this might be happening? and what can we add/remove from gateway to get this fixed.

Answer 1

Well we ended up resolving this in a strange way.
Added two gateways for the lambda
- on one of them have binary enabled
- Disabled on the other one.
For
getsomedoc - Using the one where binary media types are enabled
postsomedoc - Using the other one
Wish there was a better way!!

Answer 2

I have found this same behavior with my API. While looking everywhere for some help, I found a few things that address the issue:
Basically, this bug report says the problem is having CORS enabled while also using the generic Binary Media Type "*/*". Apparently the OPTIONS method gets confused by this. They discuss this in terms of using Serverless, but it should apply to using the console or other ways of interacting with AWS.
They link to a possible solution: you can modify the Integration Response of the OPTIONS method - change the Mapping Template's Content-Type to an actual binary media type, like image/jpeg. They say this allows you to leave the binary media type in Settings as "*/*". This is a little hacky, but at least it is something.
There also was this alternate suggestion in the issues section of this GitHub repo that is a little less hacky. You can set the content handling parameter of the OPTIONS Integration Request to "CONVERT_TO_TEXT"... but you can only do this via CloudFormation or the CLI (not via the console). This is also the recommended solution by some AWS Technicians.

Answer 3

Another possible workaround is to setup a custom Lambda function to handle the OPTIONS request, this way the API gateway may have the "*/*" Binary Media Type.
Create a new lambda function for handling OPTIONS requests:
exports.handler = async (event) => {
const response = {
statusCode: 200,
headers:{
'access-control-allow-origin':'*',
'Access-Control-Allow-Headers': 'access-control-allow-origin, content-type, access-control-allow-methods',
'Access-Control-Allow-Methods':"GET,POST,PUT,DELETE,OPTIONS"
},
body: JSON.stringify("OK")
};
return response;
};
In your API Gateway OPTION method, change the integration type from Mock to Lambda Function.
Make sure to check 'Use Lambda proxy integration'
Select the correct region and point to the created Lambda Function
This way any OPTIONS request made from the browser will trigger the Lambda function and return the custom response.
Be aware this solution might involve costs.