feat: use jpeg instead of jpg, use enums instead of strings

[mashb1t]

Reasoning:

1. we're already using jpeg to save a jpg file and should sync the filetype accordingly to support more features.
   .jpg is just a compatible form for older OS which only support 3 characters for file extensions.
2. enums allow for less typos in strings and allow the actual value behind the enum option to be changed if needed.

• https://www.adobe.com/creativecloud/file-types/image/raster/jpeg-file.html
• https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html#jpeg
• https://pillow.readthedocs.io/en/stable/handbook/image-file-formats.html#jpeg-2000