Chat: Add File Attachments demo description (#31846)

Author: vladaskorohodova

apps/demos/Demos/Chat/FileAttachments/description.md

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+DevExtreme Chat supports file attachments. When this feature is activated ([fileUploaderOptions](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#fileUploaderOptions).**uploadFile** is specified), an “Attach” button appears in the message input field, allowing users to add files to their messages. 
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+When users attach files, each file is displayed in the input area with a file-type icon, basic details (name and size), upload status, and an option to remove files before sending.
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+You can further customize the file upload process with [fileUploaderOptions](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#fileUploaderOptions) properties:
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+- **maxFileSize**    
+Specifies the maximum allowed file size.
+- **minFileSize**    
+Specifies the minimum allowed file size.
+- **multiple**    
+When set to `false`, limits uploads to a single file.
+- **allowedFileExtensions**    
+Restricts accepted file types.
+
+For the complete list of configuration options, refer to the following API section: [fileUploaderOptions](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#fileUploaderOptions).
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+The [Attachment](/Documentation/ApiReference/UI_Components/dxChat/Types/Attachment/) type includes `name` and `size` fields. To add custom fields (such as `url` in this demo), handle the [onMessageEntered](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#onMessageEntered) event and update the message object’s [attachments](/Documentation/ApiReference/UI_Components/dxChat/Types/TextMessage/#attachments) array as needed. You can use this handler to save files to your server.
+
+After a user sends a message, attachments appear in the corresponding message bubble. To allow users to download attachments, implement the [onAttachmentDownloadClick](/Documentation/ApiReference/UI_Components/dxChat/Configuration/#onAttachmentDownloadClick) event handler. You can define custom download logic within the handler.