A common mistake with JPG/GIF ads are that they are the right aspect ratio, but wrong pixel size. Lets take the ad below as an example, the name of the ad contains 320x320 but is actually 1080x1080, sometimes it’s possible to upload the bigger size in the system but more often than not your ad will not get uploaded, so here is how to check and fix that.

Inspecting the pixel size

Mac:
Scroll down below the thumbnail of the ad.

Windows:
Properties -> Information.

How to fix

Mac:

Preview the ad then follow steps below.

Windows:
Preview the ad then follow steps below.

A common problem with video is that they are too heavy or have too high bitrate, see specifications here: Video Creative Guidelines + Connected TV (CTV). Here is how to check and fix that.

Checking the bitrate

Mac:
Preview the ad then follow steps below.

Windows:
Properties -> Information.

Fixing the weight + bitrate

We at the CS-team recommend the program “Handbrake", but there is a lot of other good options for this, use your preferred.
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Drop in your video then adjust the quality here in the Video tab:

Drag the slider left to decrease the quality = less weight and bitrate, and to the right for higher quality and higher bitrate.

Make sure in the Dimensions tab that nothing gets cropped or resized unless intended:

If the video gets cropped it can help to set Cropping to None, Anamorphic to Off and Fill to None.
Here it's also possible to change the size of the creative if needed, just uncheck Optimal Size and change to desired size in Scale Size field.
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When done with the steps above you just click Start button at the top to adjust the video. Sometimes it takes a couple tries to get that perfect weight / bitrate combo, but check after every try and adjust accordingly.