Error generating Thumbnails of PDF-Files

[falk07]

This is a follow-up to #48853.

This issue seems to be still present in Version 5.6.1
Some PDF-Files cause an internal server error, some do not. (The server's error log tells something about missing headers)

Here's what i did to isolate the error:

• I examined the PDF-Files. The affected files had a PDF-Version 1.7 an were created with non-Adobe-Tools (e.g. Office365). PDFs with Version 1.4 and 1.5 (created with Acrobat) worked well.
• tried it with a local copy of the site on XAMPP: No Problems
• used a little code-snippet to prevent thumbnail-generation in general: Upload works

This is the snippet:

<?php
function disable_pdf_previews() {
    $fallbacksizes = array();
    return $fallbacksizes;
}
add_filter('fallback_intermediate_image_sizes', 'disable_pdf_previews');

[Presskopp]

Related: #45813