How many files can I upload at once?

You may upload at most 10 files one time, with a maximum total size of 100MB, or the remaining storage space for your account, if this is less than 100MB.

How do I know how much storage space I'm using?

In the upper right corner of the Files page there is a progress indicator of the amount of space you're currently using.

Some files have the text "Attached to message" in the footer. What does it mean?

When you add or edit a message, you may also attach files to that message. These documents show up on the Files page as well, but since they are tied to a message, they cannot be edited in the files page.

You may remove a file attached to a message by editing that message and deleting it from there.