This is the code for the attachmentbit template, it already has alt for the embedded images but modify it as per the second example i've given and all should be well
HTML Code:
<tr>
<td class="$bgclass">
<img class="inlineimg" src="$stylevar[imgdir_attach]/$attachment[attachmentextension].gif" alt="<phrase 1="$attachment[attachmentextension]">$vbphrase[file_type_x]</phrase>" width="16" height="16" border="0" style="vertical-align:middle" /> <a href="attachment.php?$session[sessionurl]attachmentid=$attachment[attachmentid]&d=$attachment[dateline]" target="_blank">$attachment[filename]</a>
</td>
<td class="$bgclass">
$attachment[filesize]
</td>
<td class="$bgclass" align="center">
$attachment[counter]
</td>
</tr>
Substitute all the above for
HTML Code:
<tr>
<td class="$bgclass">
<img class="inlineimg" src="$stylevar[imgdir_attach]/$attachment[attachmentextension].gif" alt="<phrase 1="$attachment[attachmentextension]">$vbphrase[file_type_x]</phrase>" width="16" height="16" border="0" style="vertical-align:middle" /> <a href="attachment.php?$session[sessionurl]attachmentid=$attachment[attachmentid]&d=$attachment[dateline]" target="_blank" alt="<phrase 1="$attachment[attachmentextension]">$vbphrase[file_type_x]</phrase>" >$attachment[filename]</a>
</td>
<td class="$bgclass">
$attachment[filesize]
</td>
<td class="$bgclass" align="center">
$attachment[counter]
</td>
</tr>