Improve the support of uploading & showing binary files

[yqs112358]

(🤔 Maybe this isn't a suggestion that fits the purpose of this project very well, but)
I've found that opengist can upload binaries (such as PDF files) via git push normally, and they're accessible after uploading. So, it might be meaningful to improve the support to binary files along the way, which is very useful to share texts with friends or to other devices along with some binary items.

Some possible improvements:

• Determine whether the file is a binary file. (Use something like https://github.com/h2non/filetype?) If so, do not display its content in the gist page to avoid jamming the browser.
• Support uploading or deleting binary files directly in the edit page of opengist. (Just add an upload button?)
• For binary files, the "Raw File" button returns the content of the file directly as application/octet-stream instead of displaying it as text.

[thomiceli]

We should discuss if we need binary files in the first place, if yes which types?
At it's core I believe Opengist should stay a simple pastebin but it can be always useful to have something like a PDF or an embedded image.

[mammo0]

• Determine whether the file is a binary file. (Use something like https://github.com/h2non/filetype?) If so, do not display its content in the gist page to avoid jamming the browser.

I think at least this point would be a very good feature. Currently I also have some PDFs in my gists and displaying them as raw text sometimes leads to a lot of scrolling.

• Support uploading or deleting binary files directly in the edit page of opengist. (Just add an upload button?)

Deleting of binary files is already possible when editing a gist. But like in the normal view they are displayed like a text file.
An upload button would be a nice feature but not a must-have.

• For binary files, the "Raw File" button returns the content of the file directly as application/octet-stream instead of displaying it as text.

I think this can be implemented together with the first point.