Unexpected behaviour of image search in SharePoint 2013

My customer has an environment with SPS2013 SP1, multiple Application Server and WFE.

We enabled search for images in image and document libraries.

Although we added the file extensions for images in search, did an IISRESET and reset of search services we had the issue hat every item in these libraries were identified as ASPX pages. After a look in the crawl logs we saw that every item was crawled by the display form and not as an image itself.

With a query tool we found out that for STS_ListItem_PictureLibrary, STS_ListItem_851 and STS_ListItem_DocumentLibrary the FileExtension was set to ASPX, FileType was empty and SecondaryFileExtension was jpg, png, gif.

We changed the managed property SecondaryFileExtension that it is searchable now.

We changed the custom refinement control (see previous post) and enhanced the template for mapping SecondayFileExtensions to FileTypes.

We changed Site Collection Result Types in Searchcenter Site Collection to point every content of our sources with SecondaryFileExtension jpg, png, gif to the Picture_Item Display Template.

Suddenly images were available including an image preview also in the hover window (again, have a look at the previous post, how to change the display templates for search items to reach this).

And suddenly again it was gone.

Weird, we found out, if you are doing a reindex on a library and starting an incremental or even full crawl, every item in this library will be crawled by the display form. If you are doing again (in our case) a full crawl, then suddenly images are crawled as images with their fileextension jpg, png, gif and everything works again.

Maybe an issue in SP1, can't imagine that this is a wanted feature. Weird!

Thanks to our technical staff and the many advices I got :)

Link to my previous post: http://sharepoint1080.blogspot.de/2014/09/problem-with-images-in-sharepoint-2013.html

Remarks:
I forgot to mention that also it seems to be a good idea, if images always have a title. Sometimes I had the feeling without a proper title the search result behaviour differs.