To test this you should set up the following: On your computer, two folders (which I will refer to as A & B), each with 3 images in (3 images in each folder can be the same, but it is helpful to have different images in the two folders), named: file.jpg file_1.jpg file_2.jpg file_3.jpg bigfile.jpg (a file larger than the currently configured max file size) A Moodle forum, with max attachments set to 3. Start a new post, then try to drag & drop the following combinations of files (reload the page, as appropriate, in between tests to reset the filemanager): 1. 'file' from A; then 'file' from B - expect options: overwrite, cancel, rename to 'file_1.jpg' 1.a) click cancel - expect: no file uploaded 1.b) click overwrite - expect: thumbnail of 'file.jpg' updated to reflect replacement file 1.c) click rename - expect: files 'file' and 'file_1' in filemanager, with appropriate thumbnails 2. 'file' from A; then 'file' and 'file_1' (at the same time) from B - expect option to rename 'file' to 'file_2.jpg' 2.a) click cancel - expect: no file uploaded 2.b) click overwrite - expect: thumbnail of 'file' updated, 'file_1' added 2.c) click rename - expect: files 'file', 'file_1' and 'file_2' all present in filemanager 3. 'file', 'file_1' from A; 'file', 'file_1' from B (at the same time) - expect options: overwrite, overwrite all, cancel, rename to 'file_2', rename all 3.a) click cancel - expect: no file uploaded 3.b) click overwrite all - expect: 'file' and 'file_1' thumbnails updated 3.c) click rename all - expect: error - you are allowed to upload a maximum of 3 files to this item (and no files uploaded) 3.d) click overwrite - expect options: overwrite, cancel, rename to 'file_2' 3.d)i) click cancel - expect: no files uploaded 3.d)ii) click overwrite - expect: 'file' and 'file_1' thumbnails updated 3.d)iii) click rename - expect: 'file' thumbnail updated, 'file_1' unchanged, 'file_2' uploaded 3.e) click rename - expect options: overwrite, cancel (no rename option, as that would exceed file limit) 3.e)i) click cancel - expect: no files uploaded 3.e)ii) click overwrite - expect: 'file' unchanged, 'file_1' thumbnail updated, 'file_2' created 4. 'file', 'file_1', 'file_2', 'file_3' from A (at same time and individually) - expect: error - you are allowed to upload a maximum of 3 files to this item (and no files uploaded) 5. 'file', 'file_1', 'file_2' from A; 'file_2', 'file_3' (at same time) from B - expect: error - you are allowed to upload a maximum of 3 files to this item (and no files uploaded) 6. 'file', 'file_1', 'file_2' from A; 'file', 'file_1', 'file_2' (at same time) from B - expect: overwrite, overwrite all, cancel 6.a) click overwrite all - expect: all thumbnails updated 6.b) click cancel - expect: no files updated 6.c) click overwrite, then cancel - expect: no files updated 7. 'bigfile' from A - expect: the file bigfile.jpg has exceeded the maximum size allowed (and no files uploaded) 8. 'file', 'bigfile', 'file_1' (at same time) from A - expect: the file bigfile.jpg has exceeded the maximum size allowed (and no files uploaded) 9. Increase the attachments limit to 8 files (or create a 'folder' resource) and then try dragging, 'file', 'file_1', 'file_2' from A; then 'file', 'file_1', 'file_2', 'file_3' from B. Click rename in each case and make sure the following renames happen: file => file_4, file_1 => file_5, file_2 => file_6 (file_3 should upload without any renaming needed) Are you still reading this? If so, congratulations! You've probably tested most situations that can occur (apart from any I've missed, so try a few other random combinations of uploading, deleting, overwriting, renaming, etc. to see what happens). Note: I've probably got some of the wording for the errors slightly wrong, as I'm writing this without looking at the code, but the gist of them should be about right