Always try and show pre rendered preview

Currently if the following situation happens

Server generates preview
Server has command removed which allows a preview to be shown
Client asks for preview, gets a 404 error when preview exists
(Mime checked before preview)

This happens more often with documents, or video as the commands are not
native PHP, they require a binary on the server.

After the fix the following would happen

Server generates preview
Server has command removed which allows a preview to be shown
Client asks for preview, gets preview which has been generated
(Mime checked after preview)

This would also allow offline generation (for example a docker image
containing the extra binaries), allowing a reduction in attack surface
of the instance serving the preview data.

Signed-off-by: Scott Dutton <scott@exussum.co.uk>

lib/private/Preview/Generator.php

@@ -125,9 +125,6 @@ public function generatePreviews(File $file, array $specifications, $mimeType =
 		if ($mimeType === null) {
 			$mimeType = $file->getMimeType();
 		}
-		if (!$this->previewManager->isMimeSupported($mimeType)) {
-			throw new NotFoundException();
-		}
 
 		$previewFolder = $this->getPreviewFolder($file);
 
@@ -154,7 +151,7 @@ public function generatePreviews(File $file, array $specifications, $mimeType =
 			$crop = $specification['crop'] ?? false;
 			$mode = $specification['mode'] ?? IPreview::MODE_FILL;
 
-			// If both width and heigth are -1 we just want the max preview
+			// If both width and height are -1 we just want the max preview
 			if ($width === -1 && $height === -1) {
 				$width = $maxWidth;
 				$height = $maxHeight;
@@ -175,6 +172,10 @@ public function generatePreviews(File $file, array $specifications, $mimeType =
 				try {
 					$preview = $this->getCachedPreview($previewFolder, $width, $height, $crop, $maxPreview->getMimeType(), $previewVersion);
 				} catch (NotFoundException $e) {
+					if (!$this->previewManager->isMimeSupported($mimeType)) {
+						throw new NotFoundException();
+					}
+
 					if ($maxPreviewImage === null) {
 						$maxPreviewImage = $this->helper->getImage($maxPreview);
 					}