Make acceptable file types for uploaded files clear

Described by @micheleidesmith in #45 (closed):

Ok I tried to upload an Excel spreadsheet after successfully uploading 2 figures, and got an eternal loading icon.

The link to 'acceptable file formats' is missing from the help here: What file formats can I use? For the manuscript text, only Microsoft Word or Adobe Acrobat can be used. For figures, tables, and supplementary files, check our list of acceptable file types.

I also get the spinning icon when I try to upload a .mov file. Not sure if that file format is allowed or not.

All of these issues are pending @parkinm et al finalising the accepted files list—the existing list is not correct (see #115 (closed)).