I personally don't use this add on although you may want to see what you have as a max upload limit on your server this can be obtained from admincp/maintence/view php info, then look on left window Configuration PHP Core/upload_max_filesize , check your value here to determine the limitations implemented at server level.
Also check the file upload limit set in the admincp to check its value admincp/attachments/attachments manager then look at the extension you where trying to upload. Hope this helps.