diff --git a/lib/unknownextensionmimeexception.php b/lib/unknownextensionmimeexception.php
new file mode 100644
index 0000000000..faa6e9cf25
--- /dev/null
+++ b/lib/unknownextensionmimeexception.php
@@ -0,0 +1,26 @@
+<?php
+if (!defined('GNUSOCIAL')) { exit(1); }
+
+/**
+ * Class for unknown extension MIME type exception
+ *
+ * Thrown when we don't know the MIME type for a given file extension.
+ *
+ * @category Exception
+ * @package  GNUsocial
+ * @author   Mikael Nordfeldth <mmn@hethane.se>
+ * @license  https://www.gnu.org/licenses/agpl-3.0.html
+ * @link     https://gnu.io/social
+ */
+
+class UnknownExtensionMimeException extends ServerException
+{
+    public function __construct($ext)
+    {
+        // TRANS: We accept the file type (we probably just accept all files)
+        // TRANS: but don't know the file extension for it. %1$s is the extension.
+        $msg = sprintf(_('Unknown MIME type for file extension: %1$s'), _ve($ext));
+
+        parent::__construct($msg);
+    }
+}
diff --git a/lib/unknownmimeextensionexception.php b/lib/unknownmimeextensionexception.php
index 0937467d07..fbc3a67742 100644
--- a/lib/unknownmimeextensionexception.php
+++ b/lib/unknownmimeextensionexception.php
@@ -17,14 +17,11 @@ if (!defined('GNUSOCIAL')) { exit(1); }
 
 class UnknownMimeExtensionException extends ServerException
 {
-    public function __construct($msg=null)
+    public function __construct($mimetype)
     {
-        if ($msg === null) {
-            // TRANS: We accept the file type (we probably just accept all files)
-            // TRANS: but don't know the file extension for it.
-            $msg = _('Supported mimetype but unknown extension relation.');
-        }
-
+        // TRANS: We accept the file type (we probably just accept all files)
+        // TRANS: but don't know the file extension for it.
+        $msg = sprintf(_('Supported mimetype but unknown extension relation: %1$s'), _ve($mimetype));
         parent::__construct($msg);
     }
 }
diff --git a/lib/util.php b/lib/util.php
index 8d95ec2305..846605bc7f 100644
--- a/lib/util.php
+++ b/lib/util.php
@@ -2000,7 +2000,7 @@ function common_supported_ext_to_mime($fileext)
 
     $supported = common_config('attachments', 'supported');
     if ($supported === true) {
-        throw new ServerException('Supported extension but unknown mimetype relation.');
+        throw new UnknownExtensionMimeException($fileext);
     }
     foreach($supported as $type => $ext) {
         if ($ext === $fileext) {
@@ -2016,7 +2016,7 @@ function common_supported_mime_to_ext($mimetype)
 {
     $supported = common_config('attachments', 'supported');
     if ($supported === true) {
-        throw new UnknownMimeExtensionException();
+        throw new UnknownMimeExtensionException($mimetype);
     }
     foreach($supported as $type => $ext) {
         if ($mimetype === $type) {