Thumbnail generation fails for files with wrong or no filename extension

[glubsy]

Problem:

• When a file has the wrong filename extension, the thumbnail is not generated properly. For example: a JPEG file has the .mp4 extension, and vice versa.
• When a file has no filename extension at all, no thumbnail is generated.

Solution:

• Detect the actual file regardless of the file extension with mime types.

This is fixed by #769