Viewnior 1.6 - Couldn't recognize the image file format (jpg file)
So.. Recently most of the downloaded from the internet images are not recognizable by the app. In this case is an normal jpg file, which can be opened by for eample Pix, XnView etc. without any problems. But not by the Viewnior. The issue also applies to other file formats like png, tiff, gif, etc. as well. Like on the screenshot..

Here is the example file..

My system: Mint 18.3 Mate x64. And here is report from inxi: inxi-report.txt
Viewnior, like Eye of Gnome (eog), is relies on gdk-pixbuf for decoding image formats. I get the following error in eog: Error interpreting JPEG image file (Not a JPEG file: starts with 0x52 0x49).
I am not that familiar with the JPEG file format myself, unfortunately. If you believe the image is correct, and this is a bug with gdk-pixbuf, please file a bug here: https://bugzilla.gnome.org/
Gimp also does not recognize this image.
As well as ImageMagick :
convert: no decode delegate for this image format '36340739-f8c021fe-13e2-11e8-96ea-5ec47b2403f8.jpg' @ error/constitute.c/ReadImage/544.
So the problem is in the file itself.
And to the owner - thank you for being made this program!
Well.. Chrome can see this pic, Pix can also see it. This default Mint image view-app has no problems with it. Or even a Gimp. And with many others pics too, they work just fine. So..
Okay.. than let's take some other one.. This for example..

No problem with second one, but format of the first image just is RIFF, but not JPEG .
Well.. and both of them are edited and saved from the Gimp app. With all "export parameters" 1 to 1 identical. So.. why one works ok and the other not? That is the question :)
RIFF actually is WEBP format, so the answer is here.
But both are jpg's.. physically speaking. Saved originally by that same camera (which does not have any riff support), on that same sd-card.. copied to the pc and there edited (just some basic cropping nothing more) and next exported trough the Gimp with that same saving settings/profiles. Yes? This is understandable? Only some of the pisc (randomly and many more as only those two) are oddly recognized by few apps as some riff's, raw's or other - internally or how should I called that - different format. Even my Phone shows normally, as jpg's, as it should be.
So.. summary.. the issue occurs only when some app uses gdk-pixbuf engine (even the latest, newest one version) and is related to the engine, not to the app direclty. I have a second system here.. Solus for example, where engine is "up-to-date-full-new" and there is similarly. On Mint is a bit older release installed because in the official repos there are no new (latest) packages available. Gimp, Pix etc.. don't use this engine and all is okay with/in them.
Okay.. for me.. this means simple: end of story :) And all I need to do is forget about Viewnior and change to other app for my daily usage. Not a problem.
First image just needs VP8 codec to be opened. Looks like your GIMP version uses that codec when exports images. The apps who do not support this codec (include my GIMP version), can't open it. That's it...
My Gimp v.2.9.9 don't have any problems. And yes.. vp8 is in chrome included and provided by, so can be that the entire system can use it. This can be true.
Opened by Gimp:

Opened by Pix:

Update: In the meantime I migrated from Mint to the Arch and the whole situation is no longer important for me. I am using now XnViewMP and it works superb for all fileformats I need. Thnx anyway :)
If anyone stumbles across this issue in the future, since that image is RIFF this issue can be solved by installing webp-pixbuf-loader.
Available on the AUR on Archlinux.