[Stream] Add filename option to Media Transformations (cloudflare#24246)

* Add filename option to Media Transformations

Adds `filename` option to Media Transformations docs and describes constraints.

* Update src/content/docs/stream/transform-videos/index.mdx

Co-authored-by: ToriLindsay <[email protected]>

---------

Co-authored-by: ToriLindsay <[email protected]>

Author: bkrebsbach

src/content/docs/stream/transform-videos/index.mdx

@@ -111,10 +111,19 @@ If `mode` is `frame`, specifies the image output format.
 
 - Acceptable options: `jpg`, `png`
 
-If `mode` is `audio`, specifies the audio output format. 
+If `mode` is `audio`, specifies the audio output format.
 
 - Acceptable options: `m4a` (default)
 
+### `filename`
+
+Specifies the filename to use in the returned Content-Disposition header. If not specified, the filename will be derived from the source URL.
+
+- Acceptable values:
+  - Maximum of 120 characters in length.
+  - Can only contain lowercase letters (a-z), numbers (0-9), hyphens (-), underscores (\_), and an optional extension. A valid name satisfies this regular expression: `^[a-zA-Z0-9-_]+.?[a-zA-Z0-9-_]+$`.
+- Examples: `default.mp4`, `shortened-clip_5s`
+
 ## Source video requirements
 
 Input video must be less than 100MB.