docs: Add entry about filenames

docs/faq.md

@@ -31,3 +31,7 @@ This error can occur when you are running tusd's disk storage on a file system w
 ### How can I prevent users from downloading the uploaded files?
 
 tusd allows any user to retrieve a previously uploaded file by issuing a HTTP GET request to the corresponding upload URL. This is possible as long as the uploaded files on the datastore have not been deleted or moved to another location. While it is a handy feature for debugging and testing your setup, we know that there are situations where you don't want to allow downloads or where you want more control about who downloads what. In these scenarios we recommend to place a proxy in front of tusd which takes on the task of access control or even preventing HTTP GET requests entirely. tusd has no feature built in for controling or disabling downloads on its own because the main focus is on accepting uploads, not serving files.
+
+### How can I keep the original filename for the uploads?
+
+tusd will generate a unique ID for every upload, e.g. `1881febb4343e9b806cad2e676989c0d`, which is also used as the filename for storing the upload. If you want to keep the original filename, e.g. `my_image.png`, you will have to rename the uploaded file manually after the upload is completed. One can use the [`post-finish` hook](https://github.com/tus/tusd/blob/master/docs/hooks.md#post-finish) to be notified once the upload is completed. The client must also be configured to add the filename to the upload's metadata, which can be [accessed inside the hooks](https://github.com/tus/tusd/blob/master/docs/hooks.md#the-hooks-environment) and used for the renaming operation.