output: Identify extension-less text types as text

See #3614

output/outputFormat.go

@@ -219,7 +219,12 @@ func (formats Formats) FromFilename(filename string) (f Format, found bool) {
 	}
 
 	if ext != "" {
-		return formats.GetBySuffix(ext)
+		f, found = formats.GetBySuffix(ext)
+		if !found && len(parts) == 2 {
+			// For extensionless output formats (e.g. Netlify's _redirects)
+			// we must fall back to using the extension as format lookup.
+			f, found = formats.GetByName(ext)
+		}
 	}
 	return
 }