Before this commit and since commit [1] which introduced
`SUPPORTED_IMAGE_EXTENSIONS` on top of the existing
`SUPPORTED_IMAGE_MIMETYPES` there would be 2 lists which were holding
related values which a strong link between the two: one was holding the
image extensions while the other was holding the images mimetypes.
It was a bad idea as it was not robust and no relation between the two.
It's an issue as some WIP code was then trying to retrieve the extension
based on a given mimetype which was not possible to do in a reliable
way:
```py
ext = SUPPORTED_IMAGE_EXTENSIONS[SUPPORTED_IMAGE_MIMETYPES.index(mimetype)]
```
The chance is then taken to merge those 2 lists in a single dict that
will strengthen the link between the mimetype and its related extension.
[1]: https://github.com/odoo/odoo/commit/9dab9ffce297dcfc89f0ec7efc4c8f8b43104220
Part-of: odoo/odoo#98801
Romain Derie
7442fb7161