By default, email attachments and browser downloads are stored at:
/sdcard/download
Can also be identified by some File Managers as:
/HWUserData/download
Some apps may have their own default location, or save to appropriated locations based on the file extension. If the device doesn't have a user-replaceable SD card the location should be:
/mnt/sdcard/Download
Some apps may have their own default location, or save to appropriated locations based on the file extension.