Fix opening PDF files with external viewers

[zerox80]

Summary

Fixes #139.

This changes the file-open intent MIME type selection so known file extensions are used when the stored MIME type is generic or missing. Concrete MIME types reported by the server are kept as-is; this keeps WebDAV metadata as the source of truth while still opening downloaded *.pdf files with application/pdf when the stored type is application/octet-stream, */*, blank, or missing.

The same MIME fallback is used in both OCFile file-open flows, and the manifest declares Android 11+ package visibility queries needed by explicit handler lookups for view/send intents.

Changes

• Add MimetypeIconUtil.getBestMimeTypeForOpen(...)
• Use the same generic-MIME fallback when opening OCFiles from both file-open flows
• Add Android 11+ <queries> entries for VIEW, SEND, and SEND_MULTIPLE
• Add unit coverage for generic, wildcard, null/blank, unknown-extension, no-extension, and concrete-MIME behavior

Validation

• ./gradlew.bat :opencloudApp:testOriginalDebugUnitTest --tests eu.opencloud.android.utils.MimetypeIconUtilTest
• ./gradlew.bat detekt
• ./gradlew.bat assembleDebug
• Verified the merged manifest contains the VIEW, SEND, and SEND_MULTIPLE package visibility queries

[guruz]

Use filename-based MIME detection when opening OCFiles

Hmm.
Why not have WebDAV return the mimetype?
Do we not know the mimetype when displaying the file list?
I'm not a fan of adding much code to re-invent stuff.

Add Android 11+ entries for VIEW, SEND, and SEND_MULTIPLE

Is this really needed? I didn't investigate it, but can't we just tell Android openThisFile("application/pdf", "/path/to/the.pdf") ?

[zerox80]

updated this to keep it more conservative:

• webdav/server mime types still win if they are specific
• extension fallback only kicks in for generic/missing types like application/octet-stream, */*, blank/null
• both ocfile open paths now use the same mime helper
• added the android 11+ queries because we call queryIntentActivities, not because startActivity needs them

also updated the pr description to match this.