Currently, when chunk=100MiB is used with a special remote, git-annex will use 100 mb ram when uploading a large file to it. The current chunk is buffered in ram.

See comment in Remote.Helper.Chunked:

- More optimal versions of this can be written, that rely
- on L.toChunks to split the lazy bytestring into chunks (typically
- smaller than the ChunkSize), and eg, write those chunks to a Handle.
- But this is the best that can be done with the storer interface that
- writes a whole L.ByteString at a time.

The buffering is probably fine for smaller chunks, in the < 10 mb or whatever range. Reading such a chunk from gpg and converting it into a http request will generally need to buffer it all in memory anyway. But, for eg external special remotes, the content is going to be written to a file anyway, so there's no point in buffering the content in memory first.

So, need something like:

prefersFileContent :: Storer -> Bool

And then when storeChunks is given such a Storer, it can then avoid buffering and write chunk data to a temp file to pass it to the Storer.