Currently, you are permitted to upload the following file types to your WordPress.com blog (no upgrade required):
See the Images help page for examples and details.
.pdf (Portable Document Format; Adobe Acrobat)
.doc, .docx (Microsoft Word Document)
.ppt, .pptx, .pps, .ppsx (Microsoft PowerPoint Presentation)
.odt (OpenDocument Text Document)
.xls, .xlsx (Microsoft Excel Document)
.zip, if you have the WordPress.com Premium or WordPress.com Business plan
See the Uploading Documents help page for examples and details.
With the purchase of the WordPress.com Premium or WordPress.com Business plans, you are also permitted to upload zip files and the following audio file types, in addition to the basic file types listed above:
There is an embedded audio player built in to WordPress.com that will work with .mp3 files.
.mp4, .m4v (MPEG-4)
.wmv (Windows Media Video)
Before uploading files to your blog, please ensure that you own, or have the legal right to distribute, them. If you do not own the files, and do not have express permission from the owner or copyright holder, you may NOT place them on your blog.