Differentiate Image Mimetypes (bmp, jpeg, gif, ico, tiff, aseprite)

[irasponsible]

Additional icons to differentiate image file type icons. This is 6 new icons (with 6 sizes each).

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It can be tricky dealing with a folder full of images with different filetypes when they have the same icon (and are too large for a preview, or unsupported by the file manager). To remedy, I made some recoloured icons to cover some basic image types; bmp, jpeg, gif, ico, and tiff. I left png as-is.

bmp	jpeg	gif	ico	tiff	aseprite	generic (unchanged)

Open to discussion or swapping any of these colours about if you have another preference! I went with orange for jpg, matching Windows XP's old icon for the same, and similar for tiff. I was originally going to use green for gif, but it was hard to find a nice looking green that differentiated well from generic.

Also added an icon for .aseprite files (using the dim purple of Aseprite's logo and UI), and additional symlinks for image-icon, image-x-icon, and image-jpeg2000. Added symlinks for 'secondary' image types such as (e.g.) portable-bitmap and image-vnd.microsoft.icon to map to image-x-bmp and image-x-ico, instead of image-x-generic.