The gitlab migration does not include the files that are uploaded in gitlab-issues. The links are converted, but the files itselfes are not transfered.
I checked some of the scripts done by other people but found none that will do that.
Because both, gitlab and gitea hide the files behind some cryptic foldernames a manual copy is not feasible.
I think technically this should be absolutely possible, because the files are accessible.
How can this be solved? Would this be an opportunity for paid support ?
I know Gitea Ltd. has supported Blender’s transition from Phabricator to Gitea, so I think they would be willing to assist you. You can contact @techknowlogick via email techknowlogick@gitea.io for more information about paid support.