Check for $this->fileInfo and @depend on testData()

lib/image.php

 			return false;
 		}
 		$this->resource = @imagecreatefromstring($str);
-		if (\OC_Util::fileInfoLoaded()) {
+		if ($this->fileInfo) {
 			$this->mimeType = $this->fileInfo->buffer($str);
 		}
 		if(is_resource($this->resource)) {
 		$data = base64_decode($str);
 		if($data) { // try to load from string data
 			$this->resource = @imagecreatefromstring($data);
-			if (\OC_Util::fileInfoLoaded()) {
+			if ($this->fileInfo) {
 				$this->mimeType = $this->fileInfo->buffer($data);
 			}
 			if(!$this->resource) {

tests/lib/image.php

 		$this->assertEquals($expected, $img->data());
 	}
 
+	/**
+	 * @depends testData
+	 */
 	public function testToString() {
 		$img = new \OC_Image(OC::$SERVERROOT.'/tests/data/testimage.png');
 		$expected = base64_encode($img->data());