Foreign character when send file image using http via multipart / form-data - http

Question

I'm trying to understand the http request, when I send a string of data using the post method on the request body there is a value that we send as below:
--------------------------d74496d66958873e
Content-Disposition: form-data; name="person"
anonymous
--------------------------d74496d66958873e
but if we send a file using post method it will be like this:
--------------------------d74496d66958873e
Content-Disposition: form-data; name="fileToUpload"; filename="icon.png"
Content-Type: image/png
-O9†q#ë#ÞÿËà3l†v}uá#t(<‡c3f
úS©59ñõCáa#Ž¡#Za%ð.ž zxý˜F#ZqÄð&^
jx[1…ÕЊËÂ$Æ‚#Þ
--------------------------d74496d66958873e
my question is:
what is the foreign character contained between -------------------------- d74496d66958873e when we send the file?
i mean
-O9†q#ë#ÞÿËà3l†v}uá#t(<‡c3f
úS©59ñõCáa#Ž¡#Za%ð.ž zxý˜F#ZqÄð&^
jx[1…ÕЊËÂ$Æ‚#Þ
whether the character is a binary, hexa, or base64 or what?
how to change image file to the character when we want to write http request manually using programming language?

Answer 1

Those are plain binary bytes. All the bytes that the file icon.png contained when it was sent by the HTTP client.
The format is described in RFC 1867 and basically works like this:
--[boundary]
[headers]
[N bytes]
--[boundary]
[headers]
[M bytes]
--[boundary]--
To extract the contents, you'll need to parse past the boundary, parse the headers (0, 1 or many) and then read the binary data until you reach the ending boundary. (The final boundary has two extra dashes on the right.)
... and there can be any amount of such parts, in a single multipart POST.