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Using the wrong mode for file uploads when using an FTP program can result in the addition or removal of characters and the file becomes corrupted. 

The two available modes for FTP transfer are ASCII and Binary. Most FTP programs automatically detect the mode that is required.

Generally speaking, all text files such as web pages, css files and scripts need to be transferred in "ASCII" mode. ASCII stands for American Standard Code for Information Interchange and it represents text in computers. 

Binary is the mode of operation for transferring non-textual data such as audio, video, and image file formats, or files that store text in code, such as PDF files and Word documents.

As mentioned, most FTP software applications are "smart" enough to detect what mode a file should be transferred in. Usually this functionality is known as "auto" or "autodetect". 

If you're using FileZilla, select "Transfer" from the menu and then "Transfer Type" to ensure it is set to "auto". If you're using a different software application, consult its help file.

Occasionally, corruption can also occur due to server timeouts and connectivity issues at the user's end - so if your FTP software is correctly configured for "auto" detection of transfer mode, exiting out of the application, then restart and attempt the upload again.