How to allow attachments in private messages and allow extension groups
In phpBB3 by default attachments are not allowed in private messages, but only in posts. If you want to allow attachments in private messages, you can do it by changing a few settings in the administration control panel.
First, log in to the administration control panel of your phpBB3 board, click on the Posting tab and then on the Attachment settings link on the left. On the Attachment settings page mark the button Yes for the option Allow attachments in private messages, and click on the Submit button to save the change. At this point, however, users still won't be able to add attachments in private messages.
On the Posting tab page of the admin panel click on the link Manage extension groups. It's on the left, under Attachment settings. You'll see a table with nine extension groups: Images, Archives, Documents, Downloadable Files, Flash Files, Plain Text, Quicktime Media, Real Media, Windows Media. Each extension group consists of different extension types. For example, the Images group has several extension types added by default: JPEG, PNG, GIFF, etc. You can add more extension types to each group. From the Manage extension groups page you can also edit and delete the existing groups, or create new ones.
Notice that under the name of each extension group it's specified whether it's allowed or not. By default, Images and Archives are allowed only in posts, the other groups are not allowed at all. You can change this by editing each group. To edit a group click on the green edit icon for the particular group. On the edit screen for the group you can allow this group in posts and/or in private messages. All the file formats that are listed as part of this extension group will be allowed as attachments.
For example, images are allowed as attachments in posts, but you want to allow them in private messages too. Click on the edit icon for the Images group, on the new page with the settings mark the checkbox for Allow in private messaging and click on the Submit button. Of course, you also have to take into account the users' permissions. By default, the standard role for registered users allows attachments in private messages. You can change this permission for any user or user group. If you need help with configuring permissions, read the tutorial on how to set and change permissions.