httr GET returns wrong content-type

httr 1.4.1 R version 3.6.1 (also tried with 3.5.3) Edit (adding verbose()) output.

I've got a request as follows:

r <- GET("https://my.cool.domain",add_headers(.headers = c('x-api-key' = 'abcdefg', 'Accept' = "text/csv")), verbose())

On my machine it responds with:

-> GET / HTTP/1.1
-> Host: https://my.cool.domain
-> User-Agent: libcurl/7.54.0 r-curl/4.2 httr/1.4.1
-> Accept-Encoding: deflate, gzip
-> x-api-key: abcdefg
-> Accept: text/csv
-> 
<- HTTP/1.1 200 OK
<- Date: Tue, 26 Nov 2019 17:50:15 GMT
<- Content-Type: text/csv
<- Content-Length: 24902
<- Connection: keep-alive
<- x-amzn-RequestId: ...
<- Content-Encoding: deflate
<- x-amz-apigw-id: ...
<- X-Amzn-Trace-Id: ...
Response [https://my.cool.domain]
Date: 2019-11-26 17:20
Status: 200
Content-Type: text/csv
Size: 209 kB
cats,dogs...
yes,no...
yes,yes...
no,no...

However on my colleague's machine (same version of httr and R, and also with an updated version of R) I get the following:

-> GET / HTTP/2
-> Host: https://my.cool.domain
-> User-Agent: libcurl/7.64.1 r-curl/4.2 httr/1.4.1
-> Accept-Encoding: deflate, gzip
-> x-api-key: abcdefg
-> Accept: text/csv
-> 
<- HTTP/2 200 
<- date: Tue, 26 Nov 2019 17:46:17 GMT
<- content-type: application/json
<- content-length: 21501
<- x-amzn-requestid: ...
<- content-encoding: deflate
<- x-amz-apigw-id: ...
<- x-amzn-trace-id: ...
Response [https://my.cool.domain]
Date: 2019-11-26 17:30
Status: 200
Content-Type: application/json
Size: 377 kB

I'm working with the developer of the https://my.cool.domain domain and I can confirm that the request header params (x-api-key and 'Accept' = "text/csv") are perfect. And the request works on my machine, and several others, but not this one colleague's.

What's going wrong here and how can I debug this?

Thanks