Attach one file to a bounty

Use the short-lived upload token returned by POST /v1/bounties. One file per request, up to 4 MB. The whole multipart body may be at most 4 MB plus 64 KiB of form overhead. Allowed extensions are CSV, TSV, XLSX, TXT, PDF, PNG, JPG, and JPEG with matching media types. A bounty accepts at most three attachments. Optionally classify the file as per_run when it changes each run or fixture when it should be reused; omit kind to leave it unclassified.