Fix asset upload: use multipart/form-data (-F) not raw binary

.github/workflows/ci.yml

@@ -161,9 +161,8 @@ jobs:
             name=$(basename "$diff" | sed 's/-diff-chromium-[a-z]*\.png//' | sed 's/-/ /g')
             response=$(curl -s -w "\n%{http_code}" -X POST \
               -H "Authorization: Bearer $VISUAL_DIFF_TOKEN" \
-              -H "Content-Type: image/png" \
               -H "Accept: application/json" \
-              --data-binary @"$diff" \
+              -F "file=@$diff;type=image/png" \
               "https://uploads.github.com/repos/${{ github.repository }}/issues/${{ github.event.pull_request.number }}/assets?name=$(basename $diff)")
             http_code=$(echo "$response" | tail -1)
             body_resp=$(echo "$response" | head -1)