Hi Ally,
sure, just change the Attachment-Template to whatever you want it to look like.
Example:
OPEN: templates/mira/topic.htm (or the topic.htm from the template you use)
FIND:
Code
<tr>
<td rowspan="5" class="cf-attach-preview-tbl uk-hidden-small"><div class="cf-attach-preview">{if $pd.system_attachment_output[atpout].a_prev && $pd.system_attachment_output[atpout].a_p_code != ''}{$pd.system_attachment_output[atpout].a_p_code}{else}<img src="upload/file_icons/{$pd.system_attachment_output[atpout].a_fimg}" alt="" title="" class="cf-attach-row-ficon" />{/if}</div></td>
<td class="cf-attach-preview-inforow"><b>{$LANG.t_filename}:</b></td>
<td><a href="attachment_processor.{$EXT}?mode=download_file&id={$pd.system_attachment_output[atpout].a_id}">{$pd.system_attachment_output[atpout].a_fname}</a><a href="attachment_processor.{$EXT}?mode=download_file&id={$pd.system_attachment_output[atpout].a_id}" class="uk-float-right"><i class="cf-text-bigger uk-icon-justify uk-icon-cloud-download"></i></a></td>
</tr>
REPLACE WITH
Code
{if $pd.system_attachment_output[atpout].a_prev && $pd.system_attachment_output[atpout].a_p_code != ''}<tr><td colspan="3"><div class="uk-text-center">{$pd.system_attachment_output[atpout].a_p_code}</div></td></tr>{/if}
<tr>
<td rowspan="5" style="width:1px;"></td>
<td class="cf-attach-preview-inforow"><b>{$LANG.t_filename}:</b></td>
<td><a href="attachment_processor.{$EXT}?mode=download_file&id={$pd.system_attachment_output[atpout].a_id}">{$pd.system_attachment_output[atpout].a_fname}</a><a href="attachment_processor.{$EXT}?mode=download_file&id={$pd.system_attachment_output[atpout].a_id}" class="uk-float-right"><i class="cf-text-bigger uk-icon-justify uk-icon-cloud-download"></i></a></td>
</tr>
Change and style however you prefer it to be.
This example will also prevent that your mobile phone will horizontally scroll the post-box.
Sincerely,
Chris
EDIT
Kenny was faster - but this will
not be a fix: Since the Attachrow is very small on mobile the behaviour to open it in default in a new browser window via download link is a CF4 feature, no bug.
So this change is basically just personal preference.
Smaller Images on the big row are also no more retina on big desktops since the image will mostly stretch to its full 100% width.
So the default behaviour is
no issue, the CF4 works there exactly how intended and tested. Beside that the CF4 Default behaviour will save your mobile users mobile bandwith (rare thing). That's why its done like this.
Sometimes Attachments are huge even though the CF4 makes thumbnail images out of it. But always gives users the decision wheather they want to download the attachment or not. That's why it was done like this. Bandwith and Screen-Size.
But if you prefer to display it always like the CF3 OVER the Table (saves space) just use my example and you're good to go.
So remember that change because it will not be integrated in the next update.