As an extract from Teambox 2.0, we're releasing our file icon set with an open-source license.
The icon set comes with vector images and PNGs in several sizes for popular file-type extensions: DMG, RAR, ZIP, TGZ, ISO, JAVA, RB, PY, PHP, C, CPP, ICS, EXE, DAT, XML, YML, SQL, ASP, H, CSS, HTML, PPT, PPS, KEY, ODP, OTP, TXT, RTF, DOC, DOT, DOCX, ODT, OTT, ODS, OTS, XLS, DOTX, XLSX, GIF, TGA, EPS, BMP, PNG, JPG, TIFF, AI, PSD, DWG, DXF, PDF, MP4, AVI, MOV, MPG, QT, FLV, M4V, MP3, WAV, AIFF, AAC, MID.
Every icon is platform agnostic and not tied to a specific program or version. This way, they will look alright for people in Windows, Mac or Linux and never feel outdated if a new version of Excel changes the icon.
Feel free to use them in your own web app, or contribute if you come up with any missing extensions!