Redesigned File Upload Example

[niftytyro]

Description

This redesigns the file upload example with better styling. Support for previewing media is added, as well as support for drag & drop.

Checklist

• [X] I have reviewed my own code
• [ ] I have added tests

[github-actions[bot]]

Size Comparison

examples	master (KB)	pull request (KB)	diff (KB)	diff (%)
boids	169.292	169.306	+0.014	+0.008%
contexts	107.345	107.346	+0.001	+0.001%
counter	84.875	84.877	+0.002	+0.002%
counter_functional	85.501	85.499	-0.002	-0.002%
dyn_create_destroy_apps	87.801	87.803	+0.002	+0.002%
file_upload	101.396	99.538	-1.857	-1.832%
function_memory_game	162.828	162.840	+0.012	+0.007%
function_router	345.488	345.473	-0.016	-0.005%
function_todomvc	157.840	157.840	0	0.000%
futures	222.037	222.035	-0.002	-0.001%
game_of_life	105.170	105.172	+0.002	+0.002%
immutable	180.707	180.709	+0.002	+0.001%
inner_html	81.973	81.977	+0.004	+0.005%
js_callback	110.686	110.692	+0.007	+0.006%
keyed_list	193.449	193.446	-0.003	-0.002%
mount_point	84.659	84.661	+0.002	+0.002%
nested_list	112.132	112.138	+0.006	+0.005%
node_refs	91.816	91.818	+0.002	+0.002%
password_strength	1547.212	1547.213	+0.001	+0.000%
portals	95.398	95.401	+0.003	+0.003%
router	314.926	314.941	+0.016	+0.005%
simple_ssr	151.352	151.347	-0.005	-0.003%
ssr_router	390.970	390.960	-0.010	-0.002%
suspense	108.411	108.413	+0.002	+0.002%
timer	87.643	87.646	+0.003	+0.003%
todomvc	139.264	139.266	+0.002	+0.001%
two_apps	85.520	85.522	+0.003	+0.003%
web_worker_fib	150.502	150.504	+0.002	+0.001%
webgl	85.655	85.653	-0.002	-0.002%

⚠️ The following example has changed its size significantly:

examples	master (KB)	pull request (KB)	diff (KB)	diff (%)
file_upload	101.396	99.538	-1.857	-1.832%

[niftytyro]

Will fix the fmt and clippy issues.

Also agreed, if we suggest using function components, the examples should use the same, and I'm anyway rewriting the example. I'll make the changes for this.

[WorldSEnder]

I've taken the liberty to push a merge of master and fix the clippy issues here. I think it's ready for review.