Currently you can upload the following file types into wordpress.com blogs. : jpg, jpeg, png, gif, pdf, doc, ppt. With the space upgrade you can also upload: mp3, avi, ogg, m4a, mp4, mpg, mov, wav, and wmv.
When uploading files (not images) to wordpress.com here are some basics worth remembering:
(1) Be sure the file you wish to upload is one of these file types.
(2) Do not use anything in the file name except letters and numbers; no spaces, no punctuation no dashes or underscores, or other special characters except for the dot between the file name and the extension. Also make sure the file name has an extension.
(3) The maximum size for a single file upload is around 8mb without the space upgrade.
http://faq.wordpress.com/2007/09/09/how-big-a-file-can-i-upload/ With the space upgrade, the maximum single file size you can upload is about 75mb. If you need more storage space, you can see the options in the “upgrades” page in your dashboard.