Media Picker - Improve UX when limiting allowed file types

[Nikkelmann]

Having tried to limit the media picker to a specific file type and gotten help from @FransdeJong in #4921 I would honestly say that this was initially confusing as there is no clear relationship between the data type "Upload File", the media type "Image" and the extensions of images, eg. jpeg, png, bmp, etc. and the documentation only says "Umbraco will automatically detect the media type and create a media item in the folder.".

The configuration points to the alias "umbracoFile" which adds to the confusion, when the example below it ties the alias of the Media Type and this configuration together: "If you need to create a custom Media Document Type to handle images called something like "Custom Image" with an alias of customImage then you need to add another object where the alias is set to customImage.".

After following the simple guide from @FransdeJong I got It working, albeit with a few gotchas:

1. The user is presented with a picker when uploading media. If they choose the wrong one, they have to delete the media and upload it again.
   There is also no guidance, so they have to know/guess what to pick.
   
   Choosing "Auto pick" will pick the Image media type, in this case where a png image is uploaded.

2. Existing images are not converted/handled, meaning applicable images have to be readded by the user with the new Media Type.

This seems like a lot of work, both the developer and for the user, for simply filtering media items by file type in a media picker.