[UCSC genome browser]custom input: variable needs to start with letter

If your file name starts with numbers or other characters, you can simply add a random letter at its head and resubmit

For example: 

12933-50mers-d16-10-ENSEMBLE-TopHat-1.2.1-mm9-single-cell-spikes.BigWig

You make it 

R12933-50mers-d16-10-ENSEMBLE-TopHat-1.2.1-mm9-single-cell-spikes.BigWig

Then the filename (variable) will be legal