feat: add file upload support for file handler (issue #854)

Add PUT method support to the file:// handler, allowing file uploads via
curl -T. Upload is opt-in, disabled by default — enable with ?put=true
metadata flag. Includes path traversal protection.

handler/file/metadata.go

@@ -7,9 +7,11 @@ import (
 
 type metadata struct {
 	dir string
+	put bool
 }
 
 func (h *fileHandler) parseMetadata(md mdata.Metadata) (err error) {
 	h.md.dir = mdutil.GetString(md, "file.dir", "dir")
+	h.md.put = mdutil.GetBool(md, "file.put", "put")
 	return
 }