Multipart Viewer isn't displaying responses when boundary has uppercase letters and/or parts with no names

### Expected Behavior

A multipart response should be displayed, with each part showing on the Select Part Dropdown.

### Actual Behavior

If the multipart response's boundary has uppercase letters in it, the viewer will give this error: Failed to parse multipart response: stream ended unexpectedly.

If I make a custom plugin to fix the casing issue of the boundary, I still have a problem with the Select Part Dropdown in cases where the response parts don't have name/filename properties. There's no default name in the component, so it populates with blanks which then get collapsed into one option. So you can't select anything past part 1.


### Reproduction Steps

Submit a request to an API that returns a multipart response. In my case it's a soap service that returns a multipart/related response that can have one or more parts.

### Is there an existing issue for this?

- [x] I have searched the [issue tracker](https://www.github.com/Kong/insomnia/issues) for this problem.

### Which sync method do you use?

- [ ] Git sync.
- [ ] Insomnia Cloud sync.
- [ ] Local only

### Additional Information

I think the issue with the content type casing is in response-viewer.tsx where the ResponseMultipartViewer is returned using the contentType variable for the parameter. That variable is lower-cased by default. I think it should have been using originalContentType instead.

For the Select Part Dropdown, the selected item and list item text just needs a fallback if the part title is null. Like: {selectedPart.title || `Part ${selectedPart.id + 1}`}

I tested both of those fixes locally and it seems to work fine.

### Insomnia Version

11.0.0

### What operating system are you using?

Windows

### Operating System Version

Windows 11

### Installation method

Download from insomnia.rest

### Last Known Working Insomnia version

_No response_

Start in response-viewer.tsx and follow the ResponseMultipartViewer entry point, checking how contentType is passed and how unnamed parts are rendered in the Select Part dropdown. Verify the viewer with a multipart response whose boundary contains uppercase letters and whose parts lack name or filename values; done means the response parses and every part remains separately selectable.

Written by the indexing model from the issue text.