Thumbnailer can't deal with german special char filenames

[benhaynes]

From directus created by airbenich : directus/directus#2205

Version Info

• Directus version and branch (Or commit hash): 6.4.9
• PHP version: 7.1.22
• MySQL version: N/A
• Web server: (Ex. Apache, nginx or IIS?) Apache
• OS name and version: N/A

Expected Behavior

Using the thumbnail for image scaling should also scale images wich contains german special-chars in the filename.

Actual Behavior

Opening a file via thumbnailer with german special chars in the filename results in the default "Image not found" graphic.

Like:
https://domain.com/thumbnail/1000/1000/contain/best/this-is-a-special-char-file-äöüß.jpg

Steps to Reproduce

1. Rename locally on your machine an image to: this-is-a-special-char-file-äöüß.jpg
2. Upload the image via directus
3. Call this image via thumbnailer:
   https://domain.com/thumbnail/1000/1000/contain/best/this-is-a-special-char-file-äöüß.jpg