By using custom file fields, you can have you visitors upload several files at once for a single topic/forum/etc. For topics and posts this isn't so important since you can already let people upload any number of files, but sometimes you may want a particular image separated from the others (for example, you can use a custom file field for a company logo while the regular upload area is used for product images). You can also add file uploads to forums and members using custom file fields.

In order to get started on creating a custom file field, visit Admin Panel -> Customizations -> Add Fields. Select the type of object (topic, forum, etc), then for "input type" select "file attachment". Note it will automatically append 'attach' to the name you give the field. Once you click 'Add Field' the template code will be added to the custom field add/edit template as usual.

Now that you have the custom file field working, you'll want to make use of it in the display (most likely in the 'show topics in forum' and/or 'main index' templates for a forum, the topic bit or topic details for topics, or the profile for a member). Here's a reference of the template variables you will have available for such display... replace 'CUSTOMATTACH' in each example with the uppercase version of the name of your custom file attachment field. These examples assume it's a forum field, replace {CAT with the appropriate prefix if it's another type of field.

{CATCUSTOMATTACHTITLE} - The name of the file for display purposes (the same name it had on the computer of the person who uploaded it).
{CATCUSTOMATTACHNAME} - The actual name on disk of the file in the attachments directory.
{CATCUSTOMATTACHDOWNLOADS} - The number of times the file has been downloaded so far.
{CATCUSTOMATTACHFILESIZE} - The size, in bytes, of the file.
{CATCUSTOMATTACHKB} - The size, in kilobytes, of the file.
{CATCUSTOMATTACHWIDTH} - For images, the width of the image in pixels.
{CATCUSTOMATTACHHEIGHT} - For images, the height of the image in pixels.
{CATCUSTOMATTACHDOWNLOADURL} - A download url which you can put in an a href HTML tag for a link or an img tag to display an image.
{CATCUSTOMATTACHTHUMBURL} - A thumbnail url which you can put in an HTML tag to display.
-- Optional parameters: width and height. Example: {CATCUSTOMATTACHTHUMBURL[500 <,> 300]} for a 500x300 thumbnail.
{CATCUSTOMATTACHTHUMBIMAGE} - The full img tag for a thumbnail.
{CATCUSTOMATTACHISIMAGE} - Determines if the file is an image. (new in 8.0)
{CATCUSTOMATTACHIMAGE} - The full image code of the thumbnail of the image. (new in 8.0)
{CATCUSTOMATTACHSLIDELINK} - A thumbnail that opens in a lightbox when clicked (images only).

To conditionalize the display to only show when there's a file attached, use ... around it. Some common examples of what you'll want to use as your display template code: