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Sorry to hear you’re facing problems 
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Some or all of the below information will be requested if it isn’t supplied; for fastest response please provide as much as you can 
Nextcloud version (eg, 12.0.2): 13.0.4
Operating system and version (eg, Ubuntu 17.04): Ubuntu 16.04.4
Apache or nginx version (eg, Apache 2.4.25): Apache/2.4.18 (Ubuntu)
PHP version (eg, 7.1): PHP 7.0.30-0ubuntu0.16.04.1
The issue you are facing:
When generating share links from the normal file view, the links look like this:
https://{DOMAIN}/s/HCd3YGtFH5Tr5Fz
When viewing the same image in gallery mode, creating a share using the icons in the upper right:
Produce a share link that looks like this:
https://{DOMAIN}/apps/gallery/s/HCd3YGtFH5Tr5Fz
This link then causes a mimetype error when being served (prompts to download the file instead of showing the share page like the first link).
Is this expected behavior?