hotfix: Support case-insensitive file extensions for drag-and-drop uploads when editing messages (#14416)

### What this PR does

Before this PR: When editing a message, files uploaded via drag-and-drop
were not recognized if their extensions were in uppercase (e.g., `.JPG`,
which is the default format on iOS). The system only performed
case-sensitive matching for allowed file types

After this PR: File extension validation is now case-insensitive.
Dragging and dropping files with uppercase extensions like `.JPG` or
`.PNG` during message editing now works as expected

Fixes #14414 

### Why we need it and why it was done in this way

On iOS and some other platforms, image extensions are frequently
capitalized by default. Our previous implementation used strict string
matching, causing valid image files to be ignored or rejected

The fix involves normalizing the file extension to lowercase during the
validation check in the drag-and-drop handler. This ensures
compatibility across different operating systems without affecting the
original filename metadata

### Release note

```release-note
fix: Support `case-insensitive` file extensions for drag-and-drop uploads when editing messages
```

Co-authored-by: SuYao <sy20010504@gmail.com>

src/renderer/src/pages/home/Messages/MessageEditor.tsx

@@ -185,7 +185,7 @@ const MessageBlockEditor: FC<Props> = ({ message, topicId, onSave, onResend, onC
     if (files) {
       let supportedFiles = 0
       files.forEach((file) => {
-        if (extensions.includes(file.ext)) {
+        if (extensions.includes(file.ext.toLowerCase())) {
           setFiles((prevFiles) => [...prevFiles, file])
           supportedFiles++
         }