Throw error in showPreview instead the constructor

This function is also used in a way such as:

```
	$preview = new \OC\Preview(\OC_User::getUser(), 'files');
	$info = \OC\Files\Filesystem::getFileInfo($file);
	if (!$always and !$preview->isAvailable($info)) {
		\OC_Response::setStatus(404);
	} else {
		$preview->setFile($file);
		$preview->setMaxX($maxX);
		$preview->setMaxY($maxY);
		$preview->setScalingUp($scalingUp);
		$preview->setKeepAspect($keepAspect);
	}
```

Which won't work anymore since `setFile` is used instead of passing the file in the constructor. Fixes a regression in master.

1 files changed, 7 insertions, 7 deletions

diff --git a/lib/private/preview.php b/lib/private/preview.php
index 21fa581d84e..292a8d481fd 100755
--- a/lib/private/preview.php
+++ b/lib/private/preview.php
@@ -111,11 +111,6 @@ class Preview {
 			\OC_Log::write('core', 'No preview providers exist', \OC_Log::ERROR);
 			throw new \Exception('No preview providers');
 		}
-
-		// Check if file is valid
-		if($this->isFileValid() === false) {
-			throw new NotFoundException('File not found.');
-		}
 	}
 
 	/**
@@ -540,10 +535,15 @@ class Preview {
 	}
 
 	/**
-	 * show preview
-	 * @return void
+	 * @param null|string $mimeType
+	 * @throws NotFoundException
 	 */
 	public function showPreview($mimeType = null) {
+		// Check if file is valid
+		if($this->isFileValid() === false) {
+			throw new NotFoundException('File not found.');
+		}
+
 		\OCP\Response::enableCaching(3600 * 24); // 24 hours
 		if (is_null($this->preview)) {
 			$this->getPreview();