Handle empty filenames as fields instead of files

[cyantree]

When crafting browser requests it doesn't seem possible to leave out the filename in case you want to send a blob:

const body = new FormData();
body.append('json', new Blob(['{"foo":"bar"}'], {type: 'application/json'}));
fetch('/test', {method: 'POST', body});

Uses "glob" as filename.

const body = new FormData();
body.append('json', new Blob(['{"foo":"bar"}'], {type: 'application/json'}), '');
fetch('/test', {method: 'POST', body});

Uses "" as filename.

const body = new FormData();
body.append('json', new Blob(['{"foo":"bar"}'], {type: 'application/json'}), null);
fetch('/test', {method: 'POST', body});

Uses "null" as filename.

Therefore such requests are handled as file because of this undefined check:
https://github.com/mscdex/busboy/blob/master/lib/types/multipart.js#L174

Is it reasonable to also accept parts with empty filenames as fields? Wouldn't a real file always have a filename?

I think that this could be seen as breaking change.

A minor change would be adding an option for a custom filtering handler which let the user decide whether a specific part should be handled as file or field.

[mscdex]

If it's any consolation, you can also use application/octet-stream as the mime type to force it as a file.

[cyantree]

Thanks for the response. Unfortunately that doesn't work with the given feature from fastify-multipart as this requires the content type to be application/json.

However Fastify forked this repo and added a config option that gives the developer the possibility to decide which part is a field or a value.
https://www.npmjs.com/package/@fastify/busboy - See isPartAFile

I will close this issue as it has been fixed in the fork.