Handle POST object upload without filename param (#6221)

POST mime/multipart upload style can have filename value optional
which leads to implementation issues in Go releases in their
standard mime/multipart library.

When `filename` doesn't exist Go doesn't update `form.File` which
we rely on to extract the incoming file data, strangely when `filename`
is not specified this data is buffered in memory and is now part of
`form.Value` instead of `form.File` which creates an inconsistent
behavior.

This PR tries to fix this in our code for the time being, but ideal PR
would be to fix the upstream mime/multipart library to handle the
above situation consistently.

cmd/handler-utils.go

@@ -17,8 +17,10 @@
 package cmd
 
 import (
+	"bytes"
 	"context"
 	"io"
+	"io/ioutil"
 	"mime/multipart"
 	"net"
 	"net/http"
@@ -235,6 +237,19 @@ func extractPostPolicyFormValues(ctx context.Context, form *multipart.Form) (fil
 		return nil, "", 0, nil, err
 	}
 
+	// this means that filename="" was not specified for file key and Go has
+	// an ugly way of handling this situation. Refer here
+	// https://golang.org/src/mime/multipart/formdata.go#L61
+	if len(form.File) == 0 {
+		var b = &bytes.Buffer{}
+		for _, v := range formValues["File"] {
+			b.WriteString(v)
+		}
+		fileSize = int64(b.Len())
+		filePart = ioutil.NopCloser(b)
+		return filePart, fileName, fileSize, formValues, nil
+	}
+
 	// Iterator until we find a valid File field and break
 	for k, v := range form.File {
 		canonicalFormName := http.CanonicalHeaderKey(k)
@@ -269,7 +284,6 @@ func extractPostPolicyFormValues(ctx context.Context, form *multipart.Form) (fil
 			break
 		}
 	}
-
 	return filePart, fileName, fileSize, formValues, nil
 }