The file formats that can be previewed or edited directly from Gmail increased a lot in the last few months, so you can't stop wondering if this is the base for an online hard drive that stores all your files and lets you edit them without having to install desktop clients.
Here's a list of the most important file formats handled by Gmail (simplified view usually means "view as HTML"):
Formats | Simplified view | Full view | Edit |
.txt | | x | |
.rtf | x | x | x |
.html | x | | |
.doc | x | x | x |
.odt (Open Office Document) | | x | x |
.sxw / .sdw (Star Office Document) | x | x | x |
.xls | x | x | x |
.sxc / .sdc (Star Office Spreadsheet) | x | | |
.ppt | x | x | |
.sxi / .sdd (Star Office Presentation) | x | | |
.pdf | x | | |
.mp3 | | x | |
.bmp/ .gif/ .png/ .jpg/ | | x | |