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So I am developing a new course-format, in which a picture is associated with each activity in a course, and presented visually. I created the course format, overrode the renderer etc. That worked all fine. However, the images are supposed to be custom generated and since it has to work for all existing and future, I put some additional code into the general course module form, enabling an image upload.

After admittedly some struggle on my part to get the File API working, it now all works fine. Only in my course format, there is an additional heading, under which you can upload a single image. This gets saved to the database fine, it is not in draft and it is viewable in my dataroots filedir perfectly if I follow the contenthash in the database. It even gets loaded into the form as a default fine. However, if I try to work with the image, all tests run fine (.is_valid_img()etc) and I even get offered to download a file. However, when I do it is corrupted and my file viewer says: "Critical Error: Not a png file". Needless to say it is not displayed on my actual course site.

When I look at the file in filedir, it very clearly is a png. Please, I would be thankful for any help, since I have tried alot and am at my wits end.

Julian Kurz
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It sounds to me like you are getting some sort of output on the page before the PNG file is sent - that would be added to the start of the file and cause it not to work as a PNG file.

I would suggest you open the file in a hex editor and check the start of the file - it should look like https://en.wikipedia.org/wiki/Portable_Network_Graphics#File_header, so look for extra characters before that.

As for where the extra characters come from - they may be an obvious warning / error message (which should be easy to track down and fix). Alternatively, you may have some stray 'echo' statements (again, fairly easy to track down). The worst problems to find are extra characters before the opening 'php' tags of a file somewhere in your install or after the closing tag at the end of a file (which is why you should never use closing PHP tags). Finding these will come down to searching through all your customised code files to locate them.

davosmith
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  • Dude, thank you SO MUCH. Yes, it was just a random var_dump, which was printed to the picture url, which I didnt take into account, because I didnt think the picture content would be generated at this place. – Julian Kurz Jun 07 '17 at 08:41
  • The picture content is not generated there, but it is loaded from the server storage and pushed out to the user's browser from there, which means there must be nothing output before the actual file content. – davosmith Jun 07 '17 at 09:04
  • Ah good to know, they should possible make this more clear on the documentation in my opinion. Anyway, thank you for your help! :) – Julian Kurz Jun 07 '17 at 09:11