AphrontFileResponse: avoid alien usages of setDownload()

Summary:
I noticed that - historically - setDownload() could also be used with
false, true, or other weird values to activate a download filename.
So, this change continues to give full compatibility to PHP 8.1 but
with extra validations.

This also adds a bit of inline documentation to put this more explicit.

For more context see the previous version:

{5952b0a31b6aac0718bc23aefe43560b9bfe8cc5}

{96ae4ba13acbf0e2f8932e950a92af0495f034d7}

Ref T15190

Test Plan:
- Drop a file in a comment, click, download
- See that it still works

Reviewers: O1 Blessed Committers, Cigaryno, avivey

Reviewed By: O1 Blessed Committers, Cigaryno, avivey

Subscribers: avivey, speck, tobiaswiese, Matthew, Cigaryno

Maniphest Tasks: T15190

Differential Revision: https://we.phorge.it/D25113

src/aphront/response/AphrontFileResponse.php

@@ -8,7 +8,16 @@ final class AphrontFileResponse extends AphrontResponse {
   private $compressResponse;
 
   private $mimeType;
+
+  /**
+   * Download filename
+   *
+   * This is NULL as default or a string.
+   *
+   * @var string|null
+   */
   private $download;
+
   private $rangeMin;
   private $rangeMax;
   private $allowOrigins = array();
@@ -18,14 +27,30 @@ final class AphrontFileResponse extends AphrontResponse {
     return $this;
   }
 
+  /**
+   * Set a download filename
+   *
+   * @param $download string
+   * @return self
+   */
   public function setDownload($download) {
+
+    // Make sure we have a populated string
     if (!phutil_nonempty_string($download)) {
       $download = 'untitled';
     }
+
     $this->download = $download;
     return $this;
   }
 
+  /**
+   * Get the download filename
+   *
+   * If this was never set, NULL is given.
+   *
+   * @return string|null
+   */
   public function getDownload() {
     return $this->download;
   }
@@ -113,7 +138,7 @@ final class AphrontFileResponse extends AphrontResponse {
       $headers[] = array('Content-Length', $content_len);
     }
 
-    if (strlen($this->getDownload())) {
+    if (phutil_nonempty_string($this->getDownload())) {
       $headers[] = array('X-Download-Options', 'noopen');
 
       $filename = $this->getDownload();