fix: Prevent unsupported file types on clipboard paste (#13182)

# Pull Request Template

## Description

This PR adds file type validation for clipboard-pasted attachments and
prevents unsupported file types from being attached across channels.

https://developers.chatwoot.com/self-hosted/supported-features#outgoing-attachments-supported-file-types

Fixes
https://linear.app/chatwoot/issue/CW-6233/bug-unsupported-file-types-allowed-via-clipboard-paste

## Type of change

- [x] Bug fix (non-breaking change which fixes an issue)

## How Has This Been Tested?

**Loom video**

**Before**
https://www.loom.com/share/882c335be4894d86b9e149d9f7560e72

**After**
https://www.loom.com/share/90ad9605fc4446afb94a5b8bbe48f7db


## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [x] I have commented on my code, particularly in hard-to-understand
areas
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [x] I have added tests that prove my fix is effective or that my
feature works
- [x] New and existing unit tests pass locally with my changes
- [ ] Any dependent changes have been merged and published in downstream
modules

---------

Co-authored-by: Muhsin Keloth <muhsinkeramam@gmail.com>

app/javascript/dashboard/i18n/locale/en/conversation.json

@@ -247,6 +247,7 @@
     "SUCCESS_DELETE_CONVERSATION": "Conversation deleted successfully",
     "FAIL_DELETE_CONVERSATION": "Couldn't delete conversation! Try again",
     "FILE_SIZE_LIMIT": "File exceeds the {MAXIMUM_SUPPORTED_FILE_UPLOAD_SIZE} MB attachment limit",
+    "FILE_TYPE_NOT_SUPPORTED": "This {fileName} file type is not supported in this conversation",
     "MESSAGE_ERROR": "Unable to send this message, please try again later",
     "SENT_BY": "Sent by:",
     "BOT": "Bot",