Also, https://github.com/mongoose-os-apps/http-fetch
If you’re not using Mongoose OS but plain Mongoose Library, you can process the MG_EV_HTTP_CHUNK message which is fired for every chunk in the upload.
Alternatively, for a non-chunked upload, use MS_EV_RECV.
Using Mongoose OS is preferrable however, because there is already a FS.* set of functions for managing files, and you can upload large files to ESP8266 with no issues.