In the latest Zotero beta, Zotero will automatically keep attachment filenames in sync with parent item metadata as you make changes (e.g., changing the title). In previous versions, while Zotero would automatically rename files when you first added them to your library, if you later edited the item's metadata, you would need to right-click on the attachment and select "Rename File from Parent Metadata".

You can configure which file types renaming applies to from the General tab of the Zotero settings.

After upgrading to this version, existing eligible files that don't match the current filename format won't be automatically renamed, but you can choose to rename them en masse from the Zotero settings. Zotero will also prompt you to rename all files if you change the filename format.

"Rename File from Parent Metadata" has been removed from the item context menu. If a filename doesn't match the configured filename format (e.g., because automatic renaming is disabled or you changed the format but didn't choose to rename all files), you can click the "Rename File to Match Parent Item" button next to the filename in the attachment's item pane to rename it.