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Client-side multipart form with the file input and upload button. This is the easy 10%: ensure the browser submits files using a proper multipart/form-data POST so the server receives file streams for further checks
The server receives the uploaded bytes and funnels them into validation. Treat this step as the place to begin rigorous checks rather than trusting filenames or claimed MIME types
Perform content-based validation by inspecting file magic numbers and actual byte signatures to confirm file type and block disguised malicious files. Reject or quarantine anything that doesn't match expected binary signatures
Persist approved files to object storage (for example S3) using generated unique names and appropriate metadata. Never place uploaded files in the public webroot; use controlled access policies and non-guessable keys
Process images (resize, convert to WebP, compress) in a controlled pipeline, then deliver stored assets from a CDN edge rather than the origin server so downloads remain fast and resilient under load