what does the leading - mean in the attachment view?

[Yubin Ruan]

Hi,

Just out of curious: in the attachment view (of the compose view), there are
some attachments with a - before them:

    -----------------------------------------------------------
    - I    1 /path/to/file1   [text/plain, 7 bit, us-ascii, 13]
    - A    1 /path/to/file2   [text/plain, 7 bit, us-ascii, 13]

what does that - mean?

Yubin