Files uploaded get corrupted in some installs storing files in database

• Error
• Community projects
• Dogfood on a *.tiki.org site
• Regression

Uploading png files to file galleries get them corrupted somehow. See screenshots (I'll upload them here in short). And a pdf file I uploaded to the file gallery, after I downloaded it again from tiki, I couldn't see its contents properly (see the other screenshot; some font seems to be missing). If I open the source pdf file (not passing through tiki file gallery but directly the one I had in my computer from elsewhere), I can see the contents of that pdf as expected

Fairly standard setup on Ubuntu 16.04 (see Ubuntu Install ), with mysql 5.7.x, and php 7.0.x (originally, but same issue with php 5.6, 7.0.x, 7.1.x, 7.2.x). Using gd (imagemagick installed also after I hit the bug the first time, just in case it automagically helped, but it didn't - I didn't remove gd, btw)

I've tested also using tiki15svn, and I couldn't use it due to some weird error I had never seen before of zend session validator class not finding Id.php in place. (!). Therefore, I couldn't test tiki15svn in the same server.

I tested using tiki17svn, and file uploads worked as expected (nice!).
I tested using tiki18svn, or tiki trunk, and I got the issue I reported.

Gosh, similar issue might be happening on dev.t.o (at least for me; is it only me???): try to see the png files that I produced as screenshots, e.g.:
https://dev.tiki.org/tiki-list_file_gallery.php?galleryId=1&fileId=1194&view=page

Update: avoided the issue by means of changing the storage of files in file galleries from db to file system.

Therefore, the issue might be specific to Tiki18+ and Mysql5.7 maybe? (in another production server of mine, the setup is fairly similar in Ubuntu 16.04, but with MariaDb instead of MySQL 5.7, and I didn't see the issue of images getting corrupted).
As a consequence, I removed the tag "Release blocker", since I'm not sure how many production server are out there with MySql 5.7 and storing files in database. Feel free anyone to re-tag to release blocker if this is too critical for someone else.

I removed "19.x" categorization because branch 19 doesn't exist yet. The category "regression from version 18 to 19" is accurate here and already assigned.

This bug has been demonstrated on show2.tiki.org

Please demonstrate your bug on show2.tiki.org