Is there a log of those who could not complete an upload?

I have a few clients that are unable to complete their uploads. Is there a place with a log that shows why the upload failed to complete?

Unfortunately there is no a way to know why a file was not successfully uploaded, maybe the file failed to upload because of the extension was not allowed, here is a guide that will help you to add or change the extensions in your file upload field: Which-file-types-are-allowed-for-the-upload-field

I have also tested your form and I can see it works fine on my end, here is what I got when I submitted it: 

Also, if possible, ask those users who are unable to upload the file to provide you with the message they're getting, that will help to know what happened to the file.

You can also find my submission in your form with the file attached, please make some tests and let us know how it goes on your end.