I understand that RAW files contain data straight from the sensor, that they're used as input to the production process that results in a final image, and that RAW isn't really one format but a collection of device-specific and mostly proprietary formats. But the fact the files are always described as "RAW" (all uppercase) rather than "raw" gives the appearance that the name is or was some sort of acronym. If that's the case, what does or did "RAW" stand for. If it's not, how did it come to be written that way?
I suspect the answer here is that the format is written as "RAW" to match other common file format names which are acronyms such as JPEG, GIF, MOV, MPEG, etc. Nikon, Canon, Pentax, Hasselblad, Olympus, Sony, and many other manufacturers all write the format in all caps, so at this point it's a de facto standard. There is a distinction to be made between a raw image file in the "RAW" sense, i.e. image data unprocessed by the device, and a raw image file in the sense of "straight from the camera, not altered using image manipulation software", and I think writing the file format as "RAW" rather than "raw" helps to make this clear. When you write it in all caps you're indicating a specific file format, even if that format changes from one device or vendor to another.
Alan Shutko points out in his comment below that older cameras used file systems that only allowed uppercase filenames, like "IMG_0001.RAW". This is an excellent point and seems a likely reason that the format was initially written in all caps.
A RAW file is a container format which includes metadata (proprietary and standard), possibly a JPEG-compressed preview, and a digitized record of the value of each sensor photosite, possibly with some low-level noise reduction applied and with dead pixels removed. This is common across the class of file formats we call RAW. Pure, "raw" sensor data alone wouldn't really be a RAW file. (And arguably wouldn't even be digital.)
People write "RAW" to make the term stand out as a graphics file format. It isn't an acronym, and it isn't even a single format, but it does mean something slightly different from the English-language word "raw" by itself. Therefore, it's useful for it to have a convention where it's written differently.
It's probably also the case that this is influenced by a tradition of writing file names and extensions in all-caps, going way back to the dawn of personal computing with DOS, when filenames were always upper-case and limited to 8 characters of the base filename plus a three-letter extension. See for example this chart comparing "DOC file" to "doc file":
or for that matter, JPEG (an acronym for Joint Photographic Experts Group) vs. JPG (the common file extension, but not an acronym) or lower-case "jpg":
You can see that even though "JPG" isn't properly an acronym, it's still more common than "jpg".
Now, there aren't, commonly, actual
.RAW files, as it instead describes a class of files each with their own actual file type and format, usually specific to a manufacturer —
.ARW, and so on, as well as the semi-standard
.DNG (which does stand for something: Digital Negative). But, nonetheless, all-caps RAW is dominant:
So, is it proper English? Nah; it's jargon. But it is a well-established convention understood by many people and is helpful shorthand which conveys extra meaning.
"RAW" is not an acronym and shouldn't be written in uppercase, it should be written in lowercase. Raw suggests that it's the image as captured by the image sensor in your digital camera or the film without any modification.
Traditionally, the filetype ".raw" was a true raw pixel format, containing only the image buffer streamed to disk with no meta data. Photoshop and other viewers would then "guess" the height, width, bit depth, and channels by matching the file size with common aspect ratios and resolutions, otherwise user interaction was required.
Now it is commonly used as a common denominator for uncompressed or even lossless compressed sensor data before bayer interpolation from the cameras incl. a lot of meta data, and jpeg thumbnails. These are not true raw files, but most of them does have a raw file inside. I say most of them, because Canon does have versions of its "CR2" format that stores half resolution "RAW" images, and these are not raw at all.
There is a useful discussion of this issue on the talk page for the Wikipedia entry for raw image format. To summarize, Wikipedia editors decided on "raw" because it is not an acronym and therefore they felt there was no logical basis for its capitalization. That said, they decided to note that it is often capitalized, acronym-style, in common usage. I wouldn't call Wikipedia authoritative but I generally follow its examples unless there's a good reason not to do so, so "raw" it is.