Uploading files fails with HTTP error 406

[UweKeim]

I'm using OpenAI Codex CLI. When trying to upload files (even very small text files) to be attached to issues/issue notes, I get e.g. this error:

Called gitlab.upload_markdown({"project_id":"153","file_path":"C:\\issue-attachment.txt"})
  └ Error: tool call error: tool call failed for `gitlab/upload_markdown`
        Caused by:
            tools/call failed: Mcp error: -32603: GitLab API error: 406 Not Acceptable
            {"status":406,"error":"Not Acceptable"}

When doing it manually via command line, it succeeds:

curl --request POST ^
  --header "PRIVATE-TOKEN: glpat-of-mine" ^
  --form "file=@C:\issue-attachment.txt" ^
  "https://<my-gitlab>/api/v4/projects/153/uploads"

and returns e.g.:

{"id":3014,"alt":"issue-attachment.txt","url":"/uploads/6693457bebceb62f3bb3f167a18eee41/issue-attachment.txt","full_path":"/-/project/153/uploads/6693457bebceb62f3bb3f167a18eee41/issue-attachment.txt","markdown":"[issue-attachment.txt](/uploads/6693457bebceb62f3bb3f167a18eee41/issue-attachment.txt)"}

An excerpt of my "config.toml" from Codex looks like this:

# ...

[mcp_servers.gitlab]
command = "npx"
args = ["-y", "@zereight/mcp-gitlab"]

[mcp_servers.gitlab.env]
GITLAB_PERSONAL_ACCESS_TOKEN = "glpat-of-mine"
GITLAB_API_URL = "https://<my-gitlab>/api/v4"
GITLAB_READ_ONLY_MODE = "false"
USE_GITLAB_WIKI = "false"
USE_MILESTONE = "false"
USE_PIPELINE = "false"

# ...

My question

Am I doing something wrong here?

Update 1

I've let Codex analyze the log files on the GitLab server:

This typically points to a missing or malformed multipart/form-data upload (e.g. an incorrect or missing file field), or incompatible request headers. The GitLab server logs unfortunately do not contain any additional details beyond the "406 Not Acceptable" response.