Feature request: file embedding (@file and =@file and :=@file)

[r-bar]

It would be great to support the file embedding syntax from httpie. From the httpie docs:

      '@' Form file fields (only with --form or --multipart):

          cv@~/Documents/CV.pdf
          cv@'~/Documents/CV.pdf;type=application/pdf'

      '=@' A data field like '=', but takes a file path and embeds its content:

           essay=@Documents/essay.txt

      ':=@' A raw JSON field like ':=', but takes a file path and embeds its content:

          package:=@./package.json