Q:

Content-Type for one file in multipart/form-data with HTTPie

I use HTTPie to POST a
multipart/form-data request (passing the -f option).   It includes a
file field (using a @ option).   The corresponding part of the
multipart request has a pseudo-header Content-Disposition, but does
not have a pseudo-header Content-Type. 
How is it possible to add such a Content-Type for a specific file? 
For completeness, here is what I send now (as shown by -p B):
--xyz
Content-Disposition: form-data; name="file"; filename="file.txt"

Hello, world! 
This is the content of the file. 

--xyz

but here is what I need to send:
--xyz
Content-Disposition: form-data; name="file"; filename="file.txt"
Content-Type: text/plain

Hello, world! 
This is the content of the file. 

--xyz

Or to put it another way, like the CURL equivalent option: -F "file=file.txt;type=text/plain". 

A:

I have just noticed that the documentation for multipart/form-data now reads like the following (at the end):

When uploading files, their content type is inferred from the file
name.  You can manually override the inferred content type:
$ http -f POST httpbin.org/post name='John Smith' cv@'~/files/data.bin;type=application/pdf'

I do not know when that happened, but it seems that now:

the "part Content-Type" is inferred automatically
it is possible to override it from the command line option

So all I was looking for! :-) 
Ref: https://httpie.org/docs#file-upload-forms.