Overview
Traditional postbacks refresh the whole page. AJAX uploads send multipart/form-data via FormData and XMLHttpRequest or fetch to handlers or Web API controllers.
Validate size, extension, and content server-side.
Implementation
Client: var fd = new FormData(); fd.append('file', input.files[0]); POST to /api/upload. Server Web API: [HttpPost] public IHttpActionResult Upload() reading HttpContext.Current.Request.Files or IFormFile in Core.
Return JSON with URL or error message.
When implementing guidance from ASP.NET AJAX File Upload, start in a controlled environment that mirrors production versions of operating systems, runtimes, and network policies. Capture a baseline before changes: export configs, snapshot VMs, or tag releases in source control so rollback stays straightforward if behavior regresses.
Document prerequisites, expected outcomes, and verification steps in a short runbook. Automated checks—smoke tests, health endpoints, or query validations—catch regressions early when platforms receive patches. Security belongs in every workflow: apply least privilege, rotate secrets, and review audit logs after deployment.
If results differ across machines, compare environment variables, permission models, time zones, and regional settings. Intermittent issues often trace to caching layers, stale DNS, or duplicated services bound to the same port.
Example
var xhr = new XMLHttpRequest();
xhr.open('POST', '/Upload.ashx');
xhr.upload.onprogress = e => console.log(e.loaded / e.total);
xhr.onload = () => console.log(xhr.responseText);
xhr.send(formData);Tips
- Scan uploads with antivirus.
- Store outside web root.
- Use chunked upload for huge files.
- Require authentication.
- Re-verify after reboots, certificate renewals, or failover exercises.
- Align monitoring and alerts with the failure modes described in this guide.
- Keep vendor documentation links handy for breaking changes between versions.
- Pair automation with a manual spot check during initial production rollout.