After upgrading to 2.10.3, my client can no longer upload anything via the File Manager - not even a vanilla 24kb jpg.
"File not Allowed" appears in red letters in the modal window where the file is chosen.
I've double-checked the mime type locally:
199701AD.jpg: image/jpeg; charset=binary
And this is obviously in-line with EE's default settings for allowed files.
I've noticed that the file is thrown away and the error is thrown after the file is uploaded. I've tried a variety of files, all have the same result. The upload directory permissions are 777 (for the purposes of debugging this.)