Need able to skip non-compatible files when combining convertible files to PDF in assignfeedback_editpdf

We found what we believe is a bug in the fileconvert plugin .

If a students uploads two files eg. a text-file and a non-convertible file, the entire pdf-conversion will fail and no file will displayed at all. 

Steps to reproduce: 

1. As teacher Create an assignment - submission type "File" and number of files allowed = 2 
2. As student upload the two attacehed files (one is a binary TI Nspire file and the other is a simple text file) in the assignet. 
3. As a teacher open the online grader for the assignment. 

Result: A blank page is shown in the grader view. 

Expected result: The txt file should be displayed in the grader viewer. 

We have implemented at fix, which is able to skip the non-compatible files option and combine/concatenate compatible files to "PDF".