A CH file is a so-called Scitex Continuous Tone file, ie a graphics file of the Scitex Workstation. The image data of a CH file is in CMYK, RGB or grayscale format and is mostly used for very large files to maintain full quality.
This format is often used on high-resolution Scitex scanners and is used in high-end prepress. In addition to the file extension CH, such files are also provided with the suffix ct, sct or sc.
Changing the extension may be useful if the file can not be opened directly.