The filename is too long, or the file is too long as in not enough room on the flash? I've not heard of a filename too long error.
When you type "show flash" from enabled mode, is the "bytes available" smaller than the file size of the new firmware? If so you'll need to delete some files, either older no-longer-used configurations, exception reports, or other files cluttering the flash.
If it's really "filename too long" you may have to open a support ticket. It's should not be necessary nor is it usually a good idea to rename firmware files.
Try using a different browser If you are using the GUI. I've had the same issue, that seems to fix the problem. I have 3 browsers on my laptop and sometimes I have to repeat the process with all 3 until one works. It seems like Firefox was the one that works best, if not try others.