Ability Office PhotoPaint
Ability Office PhotoPaint (APX) is the native format of the PhotoPaint program in the Ability Office suite, though this program also supports some other, more widely-used formats. It is apparently a different program from Corel Photo-Paint, though they both seem to be intended as replacements for Adobe Photoshop, and support the PSD format among others.
The format stores 24 bits per pixel, uncompressed, so its files are bulky but include a large amount of information in a loss-free manner. It does not appear to be widely supported or documented, so your best bet when encountering such a file would be to obtain Ability Office (a 30-day trial version is downloadable) in order to load the file and save it out in a better-supported format.
Identification
Files (at least ones in the current version as of early 2014) begin with the hex bytes 53 44 33 53, which spell SD3S in ASCII. They also have 4D 58 52 42 (spelling MXRB) at position 20 hex (32 decimal).
