The following file types would be useful to include in the Document Library, for the reasons stated:

.ics (iCalendar) -- open format utilized by all major calendar applications (desktop and mobile).  This would allow me to create a calendar event and, in the autoresponder sent when a person registers for that event, include a locally-hosted ICS file the person can use to add the event to their own calendar.

.js (JavaScript) -- given that .htm/.html are already valid in the Document Library, this is a logical addition, and will allow for better organization of custom JavaScript being used across multiple pages, allowing us to create <script> links instead of lengthy copy-pasted <script> segments on all desired pages, or the messy source that results from simply creating a PageBuilder page with a <script> element in it and embedding that where desired via [[S51]] tags.  ** Note: I'm aware this is already possible by uploading the JavaScript as a plaintext file and simply being explicit with the "type" attribute of the script tag loading it, but doing that isn't exactly good web coding.

.css -- Effectively the same as above, but for custom CSS rules that may need to be created per-page, or across multiple pages, above and beyond any CSS included in a page's wrapper.

.webm (WebM Project), .ogg (Ogg Vorbis) -- video/audio filetypes  gaining popularity (though admittedly not 100% across browser types yet) due to their licensing structures and open nature

.ttf (TrueType), .otf (OpenType), .woff (Web Open Font) -- for embedding fonts in web pages for which the uploader is licensed to use in their media

.odf, .ods, .odp, .odt (Open Office) -- sometimes we need to upload documents as templates people can edit, but not everyone has a version of Acrobat that enables editing due to the license fee, nor may they have Microsoft Office due to the same; Open Office is a free alternative, allowing the creation of rich documents without a license fee; and includes some compatibility for reading Acrobat files and reading/editing Microsoft Office files

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The following file types would be useful to include in the ImageLibrary, for the reasons stated:

.svg (Scalable Vector Graphics) -- Basic vector-graphic support, which at this point is very nearly universal across browsers.  I would love to be able to serve my sponsor's logos in their original vector format without having to generate a lot of <svg> tags from source.

This may not be the full extent of file types that would be useful; if I think of others, I will add them here.