Fix file upload filenames not being set, allow setting MIME type

[zotanmew]

This fixes uploaded files not getting the correct file_name set, and allows setting the MIME type (so that e.g. image thumbnails are generated as appropriate).

[olsh]

Thank you for the contribution.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[olsh]

Well, after some thinking, it seems like we should refactor existing files uploading instead of implementing a new method.
We should reuse existing method PostFormAsync

todoist-net/src/Todoist.Net/TodoistRestClient.cs

Lines 119 to 139 in 8f36074

	public async Task<HttpResponseMessage> PostFormAsync(
	string resource,
	IEnumerable<KeyValuePair<string, string>> parameters,
	IEnumerable<ByteArrayContent> files,
	CancellationToken cancellationToken = default)
	{
	using (var multipartFormDataContent = new MultipartFormDataContent())
	{
	foreach (var keyValuePair in parameters)
	{
	multipartFormDataContent.Add(new StringContent(keyValuePair.Value), $"\"{keyValuePair.Key}\"");
	}
	foreach (var file in files)
	{
	multipartFormDataContent.Add(file, Guid.NewGuid().ToString(), Guid.NewGuid().ToString());
	}
	return await _httpClient.PostAsync(resource, multipartFormDataContent, cancellationToken)
	.ConfigureAwait(false);
	}

But we need to improve it a bit.
We should:

1. Add mime detection to the existing PostFormAsync
2. Replace IEnumerable<ByteArrayContent> files with a custom UploadFile model that contains file name and content.