Unhelpful error message When I try to upload an image to a content type that has an image field, I get this very non-specific error: 

The file could not be uploaded. 

I have checked that the file is within the allowed size constraints, the proper format, and has no spaces or strange characters in the name. Saving the content works fine except for that one field. 

I have other fields accepting uploads and images on this and other content types and they work fine. 

I have no idea how to troubleshoot this issue. Where do I begin?
That error comes from file_managed_file_save_upload() (around line 638 of the core file.module file). 

Just before that error is a call to watchdog() with a much more descriptive error message:


  The upload directory %directory for the file field !name could not be created or is not accessible. A newly uploaded file could not be saved in this directory as a consequence, and the upload was canceled.


So it looks like it might be a permissions issue (does your web server own the sites/default/files folder?)

In case you haven't come across it before you can see you "Recent log messages" at http://mysite.com/admin/reports/dblog, it's often the best place to look when the front-end messages are unclear.