I rewrote the description since it seemed to be describing the opposite of reality, possibly due to terminology confusion between the stash and the way chunked uploads work.

As for the request itself, something like this seems like it might work:

• Add a stashedchunk or chunkfromstash (or whatever color the bikeshed should be) parameter that works just like chunk but the value is a filekey.
• When that parameter is used, load up the filekey from stash to populate $chunkPath and $chunkSize in ApiUpload::getChunkResult().
• Then at the if ( $this->mParams['offset'] == 0 ) { bit, just reuse the passed-in filekey if it's the first chunk or delete the passed-in filekey after the ->addChunk() call succeeds if it's not the first chunk.

Is this a dupe of T85561: Allow continuing one or more fully-complete uploads that are in stashed mode ?

In T112543#1638007, @Jdforrester-WMF wrote:

Is this a dupe of T85561: Allow continuing one or more fully-complete uploads that are in stashed mode ?

No. That's about looking in the stash for UI purposes, while this is about looking in the stash for new backend functionality.

Aha, I see.