content-type in FormData for string part

[BadLice]

Summary:

When performing an xhr request with content-type: multipart/form-data, it is sometimes required to specify a content-type header for a string part of the FormData. Currently, this is not possible, because the FormData only sets this header for a Blob part. This PR addresses this issue, allowing to append a part as "string object" with an optional type property that will be used by FormData.getParts as content-type headers, like it happens for Blob right now.

Changelog:

[GENERAL] [ADDED] - FormData.append function can now accept a "string object" with an optional type parameter that will be used by FormData.getParts function as content-type header

Test Plan:

before the changes:
there was no way to specify the content-type as application/json

const xhr = new XMLHttpRequest();
xhr.open('POST', 'https://v2.convertapi.com/upload');
xhr.setRequestHeader('Content-Type', 'multipart/form-data');
const formData = new FormData();
formData.append('sampleName', '{sampleProperty:"sample text value"}');

this produced the following request body:

--Q2nvVBuNTRU0yH8YBt-j_qrkfK6mVPu.8fNLbj8c4DUoBEhfgN5HuVjbZL4VaAB.vlA3k2
content-disposition: form-data; name="sampleName"

{sampleProperty:"sample text value"}
--Q2nvVBuNTRU0yH8YBt-j_qrkfK6mVPu.8fNLbj8c4DUoBEhfgN5HuVjbZL4VaAB.vlA3k2--

after the changes, it is possible to eventually specify the type of the appended part like this:

const xhr = new XMLHttpRequest();
xhr.open('POST', 'https://v2.convertapi.com/upload');
xhr.setRequestHeader('Content-Type', 'multipart/form-data');
const formData = new FormData();
formData.append('sampleName', {string: '{sampleProperty:"sample text value"}', type:'application/json'});

resulting in the following request body, with the correct content-type:

--dqHX9tecYDsMgYRgJ17SNyk62wxNEG0bnB1k-JKF9_XI4lxGUDS0u8xdx6zGr.Fmf9bHKx
content-disposition: form-data; name="sampleName"
content-type: application/json

{sampleProperty:"sample text value"}
--dqHX9tecYDsMgYRgJ17SNyk62wxNEG0bnB1k-JKF9_XI4lxGUDS0u8xdx6zGr.Fmf9bHKx--

it is still possible to pass directly a string like in the first example, the behaviour will be the same as before the changes

[analysis-bot]

Platform	Engine	Arch	Size (bytes)	Diff
android	hermes	arm64-v8a	20,460,268	+25
android	hermes	armeabi-v7a	n/a	--
android	hermes	x86	n/a	--
android	hermes	x86_64	n/a	--
android	jsc	arm64-v8a	23,658,926	+19
android	jsc	armeabi-v7a	n/a	--
android	jsc	x86	n/a	--
android	jsc	x86_64	n/a	--

Base commit: d8739e1
Branch: main