chore: support PFX filetype in attachment uploads (#14456)

# Pull Request Template

## Description

This PR expands the default upload rules to support PFX certificate
files (`application/x-pkcs12`, `application/pkcs12`, `.pfx`) across
private notes, Website, Email, and Telegram channels.

Also adds `.xls` / `.xlsx` extension fallbacks for cases where browsers
upload Excel files with an empty or generic MIME type.

### Utils Repo PR: https://github.com/chatwoot/utils/pull/61

Fixes
https://linear.app/chatwoot/issue/CW-7085/support-more-file-types-in-private-notes-and-in-app

## Type of change

- [x] New feature (non-breaking change which adds functionality)

## How Has This Been Tested?

### Screenshots
<img width="330" height="218" alt="image"
src="https://github.com/user-attachments/assets/80823250-893e-4509-adb9-61f845359151"
/>



## Checklist:

- [x] My code follows the style guidelines of this project
- [x] I have performed a self-review of my code
- [ ] 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
- [ ] 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: aakashb95 <aakashbakhle@gmail.com>

app/javascript/dashboard/components-next/icon/FileIcon.vue

@@ -18,6 +18,7 @@ const fileTypeIcon = computed(() => {
     json: 'i-woot-file-txt',
     odt: 'i-woot-file-doc',
     pdf: 'i-woot-file-pdf',
+    pfx: 'i-woot-file-pfx',
     ppt: 'i-woot-file-ppt',
     pptx: 'i-woot-file-ppt',
     rar: 'i-woot-file-zip',