How to fix SPIFFS_write error(-10010): unknown

Problem:

When trying to build a SPIFFS image, you see an error message like

SPIFFS_write error(-10010): unknown
error adding file!
Error for adding content from www!
/www/Inter-Light.27083fa6375bb9ef.woff2

Solution:

SPIFFS_write error(-10010): unknown means that the filename is too long. SPIFFS only supports filenames with a maximum of 32 characters.

In our example, Inter-Light.27083fa6375bb9ef.woff2 is 32 characters long.

Hence, the solution is to shorten the filename of every file with a filename (including extension) of 32+ characters.