We recently updated our file attachments to have a dedicated, stable URL for

downloads instead of pointing directly to the raw file URL. However, this means

that URL (FileAttachment.get_absolute_url()) no longer includes the file

extension in it.
When deciding whether to include an image of a file attachment in a message,

our chat integrations would check for file extensions in the file attachment

URLs included in a change description. Since we no longer include file

extensions in these URLs, this updates our chat integrations to grab the

file extensions from the filename.
I audited the rest of the RB Integrations, Review Board, RBTools, and

Review Bot codebases to see if we make this same assumption anywhere else,

and didn't find anything.

Ran unit tests with RB6 and RB7.
Manually tested causing chat notifications for review requests

  with an image file attachment (causing an image to appear).
Manually tested causing chat notifications for review requests

  with a text file attachment or no file attachments (causing

  no images to appear)