Gzip content-encoding with multipart/form-data - http

Question

If the post is using "multipart/form-data" content-type, and each part could be a file or other content type.
If I want to use GZIP, should the GZIP apply to the entire post body of all parts altogether or could it be possible to choose some file use gzip content-encoding while some file don't.
Is there any standard for it or only common practice ?
Thanks
For example, can I add Content-Encoding:gzip at below 'file1' part
Host: localhost:8081
Connection: keep-alive
Content-Length: 317
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.84 Safari/537.36
Cache-Control: no-cache
Origin: chrome-extension://fhbjgbiflinjbdggehcddcbncdddomop
Postman-Token: 7143164d-0da5-0e1d-112e-91f2a21c22c2
Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryPZAv0gGlJrA4ABu2
Accept: */*
Accept-Encoding: gzip, deflate, br
Accept-Language: en-US,en;q=0.9,zh-CN;q=0.8,zh;q=0.7
------WebKitFormBoundaryPZAv0gGlJrA4ABu2
Content-Disposition: form-data; name="key1"
value1
------WebKitFormBoundaryPZAv0gGlJrA4ABu2
Content-Disposition: form-data; name="file1"; filename="sample_file.txt"
Content-Type: text/plain
Content-Encoding: gzip ---------------------------IS IT OK TO ADD GZIP HERE?
This is a sample file content!
------WebKitFormBoundaryPZAv0gGlJrA4ABu2--

Answer 1

If you specify "Content-Encoding: gzip" in the HTTP response header section, it will apply to the full response.
I don't believe there's a way to apply compression to individual parts.

Answer 2

Either apply Content-Encoding compression to the whole message (end-to-end), or have the payload compressed on the transport layer, Transfer-Encoding (potentially hop-by-hop). What you can not do is compression on each part.
The multipart/form-data media type does not support any MIME header
fields in parts other than Content-Type, Content-Disposition, and (in
limited circumstances) Content-Transfer-Encoding. Other header
fields MUST NOT be included and MUST be ignored.
Emphasis mine. Source: https://www.rfc-editor.org/rfc/rfc7578#section-4.8