When creating a file by downloading a URI, truncate the length of the default name

Summary:
See <https://discourse.phabricator-community.org/t/embedding-external-images-url-show-error-for-long-urls/1339>.

When we download a file from a URI, we provide a default name based on the URI. However, if the URI is something like `http://example.com/very-very-very-....-long.jpg` with more than 255 characters, we may suggest a name which won't fit into the `name` column of `PhabricatorFile`.

Instead, suggest a default name no longer than 64 bytes.

Test Plan:
  - Used the `{image ...}` example from the Discourse report locally; got an image with a truncated name.
  - Used a normal `{image ...}`, got an image file with a normal name.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19353

src/applications/files/storage/PhabricatorFile.php

@@ -648,10 +648,17 @@ final class PhabricatorFile extends PhabricatorFileDAO
           // just bail out.
           throw $status;
         } else {
-          // This is HTTP 2XX, so use the response body to save the
-          // file data.
+          // This is HTTP 2XX, so use the response body to save the file data.
+          // Provide a default name based on the URI, truncating it if the URI
+          // is exceptionally long.
+
+          $default_name = basename($uri);
+          $default_name = id(new PhutilUTF8StringTruncator())
+            ->setMaximumBytes(64)
+            ->truncateString($default_name);
+
           $params = $params + array(
-            'name' => basename($uri),
+            'name' => $default_name,
           );
 
           return self::newFromFileData($body, $params);