The limits have been 1.14MB for jpeg and png files and 244.1K for other attachable files. We prefer that people attach files that everyone, especially Mac users, can see or use, as well as compressed formats, so we favor jpeg and png files, which web browsers support nicely, over bmp and doc files.
psd files aren't a universally displayable format, but they serve a useful purpose for graphic file exchange. gif files are supported by web browsers. zip files are used to package files for sharing. 244.1K is large for a text file. pdfs are handled nicely by Safari and Preview on Macs, but can be a bother for Internet Explorer users on PCs using Adobe Reader.
Therefore, we have increased the limits for psd, gif, and zip files to the same 1.14MB as jpegs and pngs.
Up to 1.14MB:
gif, jpeg, png, psd, zip
Up to 244.1K:
bmp, doc, pdf, txt
You can package files with smaller limits inside zips.