I am having a problem of controlling the sequence of images being uploaded (and thereby made into an attachment) on our Wordpress site so that later they appear in a specific sequence on the front end. Ordering images afterwards programmatically is not really feasible. We use a great deal of images for each post we create, and in the past the naming convention was different than now.

We used to number our images. Post title-number. But this seemed to give sometimes expected results, sometimes not.

I tried reordering the images before outputting them, but naming conventions changed, human errors were introduced, so this is not an option either. If possible I would like to save processing time and development time.

So my question is: Based on what does Wordpress decide which image to upload/turn into an attachment first in a bulk upload?