fix: apply default Content-Type in POSTObject

Browser-based POST uploads (POST /{bucket}) read Content-Type straight out
of the form fields, so a form without a content-type field stored the object
with an empty Content-Type. On read, fasthttp substitutes its own default,
so the object came back as "text/plain; charset=utf-8" rather than just
missing a type.

Fall back to defaultContentType ("binary/octet-stream") when the form field
is absent or empty, matching PutObject, CopyObject and CreateMultipartUpload,
as well as AWS S3 and Ceph RGW.

s3api/controllers/bucket-post.go

@@ -118,6 +118,9 @@ func (c S3ApiController) POSTObject(ctx fiber.Ctx) (*Response, error) {
 	parsed := utils.ContextKeyObjectPostResult.Get(ctx).(middlewares.PostObjectResult)
 	bucket := ctx.Params("bucket")
 	contentType := parsed.Fields["content-type"]
+	if contentType == "" {
+		contentType = defaultContentType
+	}
 	contentEncoding := parsed.Fields["content-encoding"]
 	contentDisposition := parsed.Fields["content-disposition"]
 	contentLanguage := parsed.Fields["content-language"]