This will slow down this a bit and bloat size, but due to react-native limitations, we have to implement this this way, because react-native will ignore any data that is not a real media file that can be read from disk src.

We can not use methods implemented in D8888 because there is no guarantee that the buffer is a valid UTF-8 string.

unused for now

This will slow down this a bit and bloat size, but due to react-native limitations, we have to implement this this way, because react-native will ignore any data that is not a real media file that can be read from disk src.

Then why can't we store those data into temporary files and pass paths to fetch API?

If this sounds like too much of a hassle then it might be worth to add a dedicated thread to CommCoreModule to perform this operation.

Mobile app already has poor performance so we shouldn't be optimistic that heavy computation we introduce to JS thread will not make it worse

this is temporary in the initial version, in future we will have to upload data from native code anyway (see ENG-4678).

in C++ or Rust, we will be able to append bytes to multipart, this is only a JS fetch API limitation.

Then why can't we store those data into temporary files and pass paths to fetch API?

The problem is fetch API in React Native doesn't understand any blobs/arraybuffers, and file paths either (besides that ph:// photo library assets). Seriously, fetch API is terrible for anything that is not a image or JSON

If we want to use temporary files, we need to use a 3rd party lib like rn-upload-blob (that @patryk is going to replace with expo-file-system)