A single file upload can not be more than 500MB (to provide bigger files, use our remotely hosted downloads feature). Also, make sure that you have enough free disk space in your E-junkie account by going to E-junkie Seller Admin > View Summary.
For best results, the original file you're uploading should have a filename using only uppercase and lowercase letters (A-Z, a-z), numerals (0-9), underscores and dashes ("_" and "-") and one dot "." only before the filename extension (e.g., .pdf, .mp3, etc.) -- try to avoid using spaces, any additional dots (besides the final extension), or any other symbols which may sometimes cause a problem. The file your buyers download may have a different filename from the original file you upload.
To ensure that your anti-virus or firewall or any other internet security software is not interfering with the upload, please disable those while you are uploading the file. This issue can also be related to your Web browser program or its settings, so if you are constantly having this issue, try using a different browser. Finally, make sure you DO NOT close the main browser window/tab in which you have E-junkie Seller Admin open, as that will log you out and abort any file upload in progress.