In this context, multimedia is being used as an adjective. Using it to describe the "content" that may be included on a website exactly matches the Merriam-Webster definition of the word as an adjective:
using, involving, or encompassing several media
American Heritage also provides a slightly more specific definition:
2. Computers Of or relating to an application that integrates different media, such as text, graphics, video, and sound.
In order to use multimedia as a noun in this context, you'd probably want to drop the following content. Multimedia as a noun seems to typically be defined as multiple types of content, as in the American English definition in the Collins English Dictionary:
a combination of media, as film, tape recordings, slides, and special lighting effects, used for entertainment or education