Unable to find encoder for "spdx-json=spdx.json"

[wieringen]

What happened:

What you expected to happen:
I'm trying to sign and generate a SBOM and output it to a file. One of the options syft attest --help tells me exist is:

-o, --output stringArray report output format (<format>=<file> to output to a file), formats=[cyclonedx-json cyclonedx-xml github-json spdx-json spdx-tag-value syft-json syft-table syft-text template] (default [syft-json])

But when I try this option, it outputs the following error:
ERROR unable to write SBOM to file: unable to find encoder for "spdx-json=spdx.json"

I looked in the source and this behavior doesn't seem to be implemented. Any thoughts?

Steps to reproduce the issue:
go run cmd/syft/main.go attest DIGEST --key=KEY -o spdx-json=spdx.json

or

syft attest DIGEST --key=KEY -o spdx-json=spdx.json

[kzantow]

As I understand it, when you use the syft attest command, it invokes cosign and pushes the signed SBOM w/attestation alongside the image using the same OCI registry. This means there really aren't files on the filesystem that get produced, so outputting to a file isn't implemented today. But it seems like a reasonable request to output the contents that would otherwise get pushed to the registry to a specified file -- is this what you are hoping to do?