fix: invalid multipart/form-data (#948)
fix: invalid multipart/form-data (#948)

According to https://tools.ietf.org/html/rfc7578#section-4.2, in a Content-Disposition header
of a part in a multipart/form-data body:

- the disposition must be of type `form-data`
- a `name` parameter (not explicitely specified by the RFC, but I suppose non-empty)

As this `name` parameter is unused by IPFS, this commit simply generate a placeholder with
a counter, to conform to the HTTP specification.
ipfs/js-ipfs-api