fix: sending formdata bodies with http2

[KhafraDev]

refs: #3803 (comment)

[github-actions]

The backport to v6.x failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-v6.x v6.x
# Navigate to the new working tree
cd .worktrees/backport-v6.x
# Create a new branch
git switch --create backport-3863-to-v6.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 e49b5751ddfe726ebc6498f07a4af86de319b691
# Push it to GitHub
git push --set-upstream origin backport-3863-to-v6.x
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-v6.x

Then, create a pull request where the base branch is v6.x and the compare/head branch is backport-3863-to-v6.x.