We are using an upload class which detects the MIME type of the file and reads the first bytes of that image.

The upload will fail if:
a) the MIME types are not properly configured in your server
b) the actual image has been corrupted.

In the DEV - github.com/getk2/k2 - version the class has been updated but no drastic changed have been applied so that functionality will be altered.

Can you try the following?
a) Use the DEV version
b) Use a different image, eg: a png or an existing image and/or
c) C/p our conversation to your hosting provider.
d) And of course let me know how it went.

Hi Kriker. This comes from www.joomlaworks.net/forum/k2-en/45116-solved-image-upload-error-k2-7-and-j3-5/reply/157120#157205.

I have the image issue in backend (have not checked whether the issue replicates in frontend). Browse server functionality does not work either.

* Config: Joomla 3.6.2 / PhP 5.6.22 / K2 2.7.1
* No overrides
* No K2 extensions
* Checked MIME types server config (ok)
* Used DEV version
* Tried uploading several types of images: jpg, png, gif. It fails uploading any of them.

Unfortunately the issue is still there...

Are you on Apache/ Nginx?
Do you know your server's OS?

I 'll also report it on Github so we can get feedback there as well.

Sorry for the late reply: Apache 2.4.6 running on RHEL. 7.2.

Incidentally (I guess) I have recently performed a migration from an nginx server. Unfortunately, at this point I am not sure whether the problems started with the migration or after updating to the k2 2.7 version...

To be honest I've tried multiple versions of PHP and multiple combinations.

They all seem to work as intended (thankfully but bummer).
Does the backend and the media manager work properly?

Yes. Everything but this :-( seems to be ok... Thank you so much for taking care of this, anyway.

Today I'll talk on the phone with our hosting provider. I'll ask them again to get PHP updated to v7. It seems that having PHP updated to v7 has been a work-around for everyone with this issue.

PHP 7 is an improvement in terms of speed as well.
Take a snapshot of your site before updating.

Furthermore in case you are using AllVideos, the DEV 4.8.0 version resolves issues with PHP7 compatiblity.
github.com/joomlaworks/allvideos

I am coming back to my issue of not being able to upgrade my site to K2 2.7.1 without wiping out some of my menu items. I do have installed a JCE editor Add On called "K2 Links for JCE Link"

I think that is a tool developed by you all. I have on my system a version that is:
K2 Links for JCE Link Links 2.2 31 January 2013
This is ahead of the version number which I can download from the Getk2.org web site.
The version available there is: Extension Version: 2.0.1

I am thinking that there was some other problem on my site previously and that one you you might have provided me the updated version to fix that. I am assuming that possibly that might be the cause of my problem. Mostly I am putting this here to document my issue before I play with it again.

I am having the problems at www.allmedphysicians.com/

Sean

The easiest way would be to perform a semi-manual update.

a) Backup your site and db using Akeeba Backup
b) Backup the menu table. ##__menu.
c) After the update replace the current menu table (which is missing entries) with the old menu table which is complete.

We're having the same issue since trying to set up a frontend login & entry system for an existing site.

If you use the frontend form, AND YOU HAVE FRONTEND IMAGES DISABLED (in K2 parameters -> Frontend Editing -> Image tab on edit form OFF), you get the message after saving on top:
The image was not uploaded. Please ensure that you used a valid image file and that file uploads are properly configured on your server.

It's probably some sort of false positive check, since the image can't be "invalid" if it's never picked, nor is the code to offer the choice never generated.

When we turn on the ability to select an image on Frontend, we don't get the message, but we don't want to allow the option in our current case.

The site is running Joomla 3.6.2, K2 2.7.1. Backend works fine - images and all. It's just the frontend (if set as we have) that's the issue.

We've tried with PHP 5.6 and PHP7 - both give us the error. We've tried with and without our modifications to the template.

We're running with no additional K2 mods installed.

Krikor, in case you'd want to investigate I cannot give you credentials to the live site since it's a client's site, but if you want I can clone the site to another location (but the same server) and wipe all the data relating to our customer - just keep the skeleton, current state and dummy text...

(On another note: K2 2.7.1 doesn't like Ark Editor as the frontend editor for the Extra Fields - the main window works fine but the Extra Fields give you some... editor look I've never seen before, and when you try to save it behaves as if there's nothing written, as if it's empty - just a heads up)