Remove the curly brackets around NEXUS_UPLOAD and use -x to enable the trace option, and don't forget to "export" the variables.
You can see a list of exported variables with declare -p
#!/bin/bash -x
for i in $(cat upload.list); do
content=$(curl -v -u "$NEXUS_USER":"$NEXUS_USER_PASSWORD" --upload-file "$i" "$NEXUS_UPLOAD")
echo $content
done
Here's a link explaining when they are useful.
For example:
i=image.jpg
convert $i ${i%jpg}png
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