Handle files with is_file instead of file_exists

Should fix things like `fread(): read of 8192 bytes failed with errno=21 Is a directory`

lib/private/Files/Storage/Wrapper/Encryption.php

@@ -389,7 +389,7 @@ public function fopen($path, $mode) {
 		if ($this->util->isExcluded($fullPath) === false) {
 			$size = $unencryptedSize = 0;
 			$realFile = $this->util->stripPartialFileExtension($path);
-			$targetExists = $this->file_exists($realFile) || $this->file_exists($path);
+			$targetExists = $this->is_file($realFile) || $this->file_exists($path);
 			$targetIsEncrypted = false;
 			if ($targetExists) {
 				// in case the file exists we require the explicit module as
@@ -853,7 +853,7 @@ protected function getFullPath($path) {
 	 */
 	protected function readFirstBlock($path) {
 		$firstBlock = '';
-		if ($this->storage->file_exists($path)) {
+		if ($this->storage->is_file($path)) {
 			$handle = $this->storage->fopen($path, 'r');
 			$firstBlock = fread($handle, $this->util->getHeaderSize());
 			fclose($handle);
@@ -870,7 +870,7 @@ protected function readFirstBlock($path) {
 	protected function getHeaderSize($path) {
 		$headerSize = 0;
 		$realFile = $this->util->stripPartialFileExtension($path);
-		if ($this->storage->file_exists($realFile)) {
+		if ($this->storage->is_file($realFile)) {
 			$path = $realFile;
 		}
 		$firstBlock = $this->readFirstBlock($path);
@@ -918,7 +918,7 @@ protected function parseRawHeader($rawHeader) {
 	 */
 	protected function getHeader($path) {
 		$realFile = $this->util->stripPartialFileExtension($path);
-		$exists = $this->storage->file_exists($realFile);
+		$exists = $this->storage->is_file($realFile);
 		if ($exists) {
 			$path = $realFile;
 		}