Support multiple variants of export file format names in inx files
#### Summary:
When trying to save as jpg, the new raster output extension only allows the precise word 'jpeg'. At least, it's case-insensitive, so one can also use JPEG.
It would be useful to allow multiple extensions.
For the time being, I've worked around the problem by creating a second inx file.
#### Steps to reproduce:
- open Inkscape
- Save as ...
- Select 'guess from extension'
- type /tmp/file.jpg
#### What happened?
The file was saved as file.jpg.svg
#### What should have happened?
The jpg export dialog should pop up.
#### Version Info:
```
- Inkscape Version: 1.1-alpha (da7a25772f, 2021-02-09)
- Operating System: Linux Mint 20.1
```
issue