function file_file_update has a case-sensitive check of the file's mime type

This causes issues for me because my file has a mime type of "APPLICATION/PDF"

So I changed the switch statement to this and it started working

switch (strtolower($file->filemime))  {

The symptoms of this issue where that the fileviewer iframe was showing but not loading the pdf files. Behind the scenes the pdf files were not being generated and the following error was being logged:

Notice: Trying to get property of non-object in include() (line 28 of [PATH TO SITE]/modules/fileviewer/theme/bookreader-viewer.tpl.php).

Similar symptoms to #1062696: Files not rendering/showing and #1354806: trying to get property of non-object in line 28 of fileviewer/theme/bookreader-viewer.tpl.php