Paint Shop Pro supports about 50 file formats, including BMP, CDR, CGM, CLP, CMX, CUT, DCX, DIB, DRW, DXF, EMF, EPS+, FPX, GEM, GIF, HGL, IFF, IMG, JPG, KDC, LBM, MAC, MSP, PBM, PCD, PCT, PCX, PGM, PIC, PIC, PNG, PPM, PSD, PSP, RAS, RAW, RLE, SCT, TGA, TIF, WMF, WPG, WPG.........
but not .MIX which is a non-standard format that can only be used by Microsoft products like MS Picture It, MS Digital Imaging Pro, MS PhotoDraw and MS Word, and possibly some earlier versions of Corel Draw.
Paint Shop Pro features and file support
The .MIX format is similar to the more commonly used .PSD format (Photoshop, Paint Shop Pro, etc) in that it stores the elements of an image as separate entities rather than a single flat image. .PSD stores these elements as layers, text objects, selected areas, etc. which makes it ideal for editing purposes before finally saving as TIF or JPG.
MS Digital Imaging Pro supports .MIX, but unfortunately Microsoft appears to have produced more than one version of the format, so files saved in one program may not work in another. In other words, it's not a very good file format and should be avoided.
ReaConverter ($50) is the only program I can find that can convert .MIX files, but I haven't used it so I can't say how good it is, and you may need the Pro version ($100).