dsh-file-fix: Adding unified file import for DSH Web

# Introduction If you have tried uploading files on DSH Web, you have likely encountered these issues: dragging in non-image files triggers a prompt saying "Only PNG, JPG, WebP, GIF are supported"; the "Drop Image" overlay gets stuck and does not disappear after a failed drag; the input box lacks a file selection button; and pasting files with Ctrl+V has no response. Even when files are already on the server, the agent cannot retrieve a file list and is forced to repeatedly guess the path—a README example recorded an instance where the agent took 75 steps of guessing. `dsh-file-fix` solves exactly this chain of problems. It unifies file import into a single method: files with any extension can be dragged in, pasted, or selected via click, with their bytes uploaded directly to the attachment library. The file list is injected into the model's context along with the message and is displayed below the history message and can...