What is a word for a file that has never been compressed?

I think you have to remember what the prefixes mean. Checking with online dictionaries reveals that the prefix un- basically means not. So uncompressed doesn't necessarily imply that it has been compressed before. It just means that it's simply not compressed.

De- however signifies removal or reversal, and so decompressed does imply that a file was once compressed.

For image files specifically, a raw file is one that has had as little processing as possible since being captured by a camera's sensor. There are a lot of different raw formats for image files, they're often specific to a given brand or model of camera.

Often you need to use special settings on a digital camera to get access to raw image files. Most cameras will, by default, apply some low-level processing before exporting images in a format more easily viewable and editable, while raw images are only useful for specialists.

Compression is one form of processing, though lossless compression (that you can reverse without losing any data) isn't usually something to be concerned about. On the other hand, lossy compression is a very destructive kind of processing, as it introduces artifacts into the image that you can't easily get rid of. Repeatedly compressing and decompressing an image with a lossy algorithm (like the one used for .jpeg files) will rapidly destroy its quality.

There's not really an equivalent for most other kinds of files on a computer (except maybe audio or video, which can use similar lossy compression methods). Copying a generic file or compressing and uncompressing it using a lossless algorithm doesn't generally change the file in any notable way, so it's not a thing that needs special jargon. Maybe you'd lose some metadata (like the date the file was created), but often not even that.

So in general, there is no specific word for the situation you are asking for. If you hope that your reader will understand and be able to recognize a distinction between an uncompressed and a decompressed file, you're very likely to be disappointed. Most computer savvy people will expect those words to be synonymous. Even if you use an unambiguous compound adjective like never compressed, many readers will be perplexed about why you think that is significant.